In truth, most users will not notice the disappearance of circuit-switched voice, writes Brendan Kearns. They already rely on VoIP, VoLTE, and messaging apps without thinking about the underlying transport. For engineers, however, the closure of the last circuit-switched networks will mark the end of a century-long era – a symbolic milestone in the story of telecommunications. Circuit switching will survive in textbooks and museums, remembered as the technology that first enabled the world to speak.
The rise
When Alexander Graham Bell demonstrated the telephone in 1876, his invention was essentially an instrument – a pair of devices capable of converting the human voice into electrical energy and reproducing it at a distance.
At that stage, telephony was strictly a point-to-point arrangement, requiring each pair of telephones to be directly connected by their own dedicated wire circuit. The concept of switching, signalling, and network interconnection had not yet emerged, for these systems evolved later out of necessity.
By contrast, the telegraph – developed several decades earlier – had already established the science of transmission, including the electrical behaviour of long wires, resistance, inductance, and the use of relays for distance extension. Thus, when the telephone appeared, the underlying principles of electrical transmission were already well understood; what remained to be invented were the means to make telephony scalable.
Invention of the telephone
When one reads about the invention of the telegraph, there are many paths and many teams – on both sides of the Atlantic – racing towards an inevitable conclusion. The telegraph was in great demand; society was ready for instant communication, and its arrival was only a matter of time.
The telephone, however, was different. Few were working on it, many dismissed it as a curiosity or even a humbug, and its value had to be painstakingly explained and sold by Bell after its invention. It is fitting, then, that the telephone’s birth was not a calculated race but an accident of discovery – two separate mishaps in the laboratory that brought about one of the most transformative devices in human history.
Alexander Graham Bell was working on what he called the 'harmonic telegraph'. This device used tuned reeds, like those in a harmonica, to allow multiple telegraph messages to be sent simultaneously over the same wire.
One day, in June 1875, while experimenting with this system, Bell’s assistant Thomas Watson accidentally plucked a reed and Bell heard the sound through the apparatus. This revealed that not just electrical pulses, but the vibrations of the human voice could be carried over a wire. This changed the direction of Bell's investigations and he started to work towards the first telephone instrument.
Bell and Watson worked through 1875 and early 1876 and Bell needed to hide this from his investors who regarded the phone as a foolish pursuit. Bell’s patent application filed in February 1876, was based on a liquid transmitter and, granted on March 7, 1876, was written in very broad and somewhat ambiguous language. It described an “improvement in telegraphy” rather than explicitly naming a “telephone”.
The key phrase referred to the transmission of “vocal or other sounds telegraphically”, which was vague enough to cover the general concept of transmitting speech by electricity – even though the technical details were not fully worked out. The document effectively claimed the general principle of electrical voice transmission, not just a specific mechanism – which gave Bell the legal foundation for the invention of the telephone.
The second breakthrough came on March 10, 1876, when Alexander Graham Bell accidentally spilled acid on himself and called out, “Mr Watson, come here, I want to see you”. To his astonishment, Thomas Watson, in another room, heard the words clearly through the experimental transmitter – the first intelligible sentence ever transmitted by telephone.
After securing his first telephone patent on March 7, 1876, Alexander Graham Bell submitted a second, more detailed patent later that year. Filed in April 1876 and granted in January 1877 as US Patent No 186,787, it was titled 'Improvement in Electric Telephony'.
While the first patent broadly described the principle of transmitting speech electrically, the second focused on practical refinements – notably improved transmitter and receiver designs that used magneto induction rather than the earlier liquid transmitter. These improvements made the telephone stable, reliable, and suitable for commercial use.
Figure 1‑1: Bell's liquid transmitter and the first two phones.
Switching
Two years after invention of the phone, in 1878, the concept of circuit switching emerged, creating the foundation for the telephone network. Unlike the telegraph, which required no switching, telephony demanded a mechanism for connecting any subscriber to any other on demand.
In the telegraph era, a message sent from Dublin to America would be manually relayed through intermediate stations – for instance, transmitted from a post office to a hub in Kerry, then recreated by an operator for onward transmission across the Atlantic, where another operator in Newfoundland would repeat the process, and so on. Such manual regeneration was impossible for a continuous voice signal, so a new idea was required: the telephone switch.
The evolution of switching technology over the following century can be divided into four distinct phases.
- The first, the manual era (1880-1910), depended on operators to establish connections by hand;
- The second, the electromechanical era (1910-1970), saw the introduction of the Strowger automatic switch and later the crossbar system, representing the first major technological leap in automated telephony;
- The third, the digital era (1970-2010), began with the introduction of digital switching and time-division multiplexing, continuing through to the gradual closure of the analogue telephone network;
- The fourth and current phase, the packet-switched era (from 2010 onwards), marks the deployment of IP-based systems such as the IP Multimedia Subsystem (IMS), which replaced traditional circuit-switched voice with packet-switched communication and unified fixed and mobile telephony into a single all-IP environment.
Manual switching
When a customer lifted the receiver, the operator would plug a cable into the correct jack to link the caller with the person they wanted to reach. This worked reasonably well for local calls, but long-distance connections – known as trunk calls – were much more complicated.
For these, a regional operator had to contact other operators in different towns or cities, each of whom connected a section of the route until the full path was completed. As more people began using telephones, particularly in cities, this manual process became increasingly slow and inefficient.
Figure 1‑2: transit calls.
Electromechanical switching
Almon B Strowger, an undertaker in a small Midwestern town, invented the first automatic telephone switching system in the 1890s. His motivation was personal: his competitor’s wife worked as a telephone operator and, according to legend, diverted business calls to her husband instead of him.
To remove this human interference, Strowger devised an electromechanical system that allowed subscribers to connect directly without operator assistance.
Figure 1‑3: Step-by-step switch – direct control.
In a step-by-step (Strowger) exchange, the connection between subscribers was established through electromechanical means, controlled directly by the pulses generated from the caller’s rotary dial.
When the user turned and released the dial, it interrupted the DC line current in a series of timed pulses – one for each digit dialled. These pulses energised an electromagnet in the selector switch, causing it to move step by step through a set of contacts.
The later crossbar switch introduced a significant change in this process. Instead of being directly driven by the subscriber’s pulses, the digits were first received and stored by a common control unit, which then determined the most efficient path through the network.
The crossbar system used horizontal and vertical bars to create connections via magnetic latching, allowing faster call setup, reduced mechanical wear, and greater flexibility compared with the purely pulse-driven step-by-step system.
Digital switching
The introduction of digital switching in the 1960s and 1970s marked another significant turning point in telephony. Earlier electromechanical systems such as crossbar relied on physical contacts and relays to establish a circuit between two subscribers.
Digital switching replaced these moving parts with electronic logic, using Pulse Code Modulation (PCM) to convert analogue voice signals into streams of binary data. Each call was assigned a time slot within a digital frame, allowing multiple conversations to share the same transmission path through time-division multiplexing (TDM). This dramatically improved efficiency, reliability, and maintainability, while also reducing power consumption and physical space in exchanges.
At the heart of digital switching lay the principle of space-time switching. Traditional 'space' switching involved connecting specific physical paths between lines, while 'time' switching rearranged the sequence of digital time slots to route calls correctly.
By combining these two dimensions – space and time – digital exchanges could handle thousands of simultaneous connections entirely in software-controlled memory rather than through mechanical motion. In the depiction below we have a call coming in from left on System 1/ Time Slot 3 switched to 6/6 and 3/7 switched to 8/6 in the other transmission direction.
Figure 1‑4: Space and time switching: (Each call has transmit & receive).
Systems such as the Alcatel E10, Ericsson AXE, and Nortel DMS exemplified this new architecture, which quickly replaced electromechanical crossbar systems worldwide. Digital switching not only modernised the Public Switched Telephone Network (PSTN) but also laid the foundation for integrated voice and data services and the eventual transition to all-IP networks.
ISDN and GSM
During the early 1990s, the European Telecommunications Standards Institute (ETSI) developed two big standards that would shape the future of telecommunications: ISDN and GSM.
Although conceived around the same time, their long-term impact was very different. Integrated Services Digital Network (ISDN) was introduced in Ireland in 1994 as part of the 'EURO-ISDN' initiative, representing the peak of the traditional telephone network’s capabilities.
It enabled the simultaneous transmission of voice and data over a single digital line, offering clear audio quality and faster data connections than analogue modems. For a brief period, ISDN symbolised the cutting edge of digital telephony – the zenith of the circuit-switched era. However, its success was short-lived. As internet-based, packet-switched networks grew in popularity and broadband became widespread, ISDN usage declined steadily, culminating in its shutdown in 2024 after three decades of service.
In contrast, GSM (Global System for Mobile Communications) became one of the most influential telecommunications standards in history. It unified the fragmented landscape of national analogue mobile systems into a single, interoperable digital platform.
GSM introduced encryption for secure communications and the Subscriber Identity Module (SIM) card, which allowed users to switch easily between devices while retaining their personal identity and number. Its combination of reliability, scalability, and roaming capability transformed mobile telephony into a truly global service. Even today, GSM remains the default fallback for voice calls when roaming internationally – a testament to the enduring strength and universality of the standard.
Packet switched voice
By the early 2010s, the IP Multimedia Subsystem (IMS) had become the global standard for delivering voice and messaging services over modern data networks. It replaced the old circuit-switched systems that had powered telephony for more than a century, bringing all communications – voice, video, and text – onto a single, packet-switched IP platform. Instead of relying on dedicated connections for each call, IMS uses flexible internet-based routing and standardised control functions to manage sessions across any access network, whether mobile, fixed, or Wi-Fi.
Voice over LTE (VoLTE) and Voice over Wi-Fi (VoWiFi) are the most visible outcomes of this shift. Both carry voice as digital data, using the same IP infrastructure as other internet traffic but with guaranteed quality and priority. VoLTE allows mobile calls to run natively over 4G networks without falling back to older systems, while VoWiFi extends the same service over home or public Wi-Fi connections. Together, they mark the final step in the long transition from circuit-switched telephony to a fully all-IP voice world.
Switching in Ireland
Alexander Graham Bell had demonstrated the telephone to the Post Office in 1877, and the chief electrician had urged the department to secure manufacturing rights. However, by the time government approval came through, it was too late – the National Telephone Company had already obtained the necessary licences from Bell. Ireland’s first telephone exchange was established in 1880 on the top floor of the Commercial Buildings on Dame Street, Dublin, operated by the United Telephone Company. This early manual switchboard served a mere five subscribers.
By 1881, the exchange employed its first female operator, Miss Agnes Duggan. At that time, the staff consisted of a manager, three clerks, the lady operator, and a handyman who acted as fitter, electrician, and linesman, assisted by a young boy who carried the tools in a straw basket on his back.
The number of subscribers had grown to about 20. The switchboards themselves were very basic, and operators often had to ring back customers to ask whom they wished to call. Continuous service was not yet available, and the exchange operated only between 9am and midnight.
It took 50 years, and the first automatic telephone switch in Ireland was installed on Ship Street in 1927. A mere three years later, by 1930, Dublin had become one of the first cities in the world to achieve full local automation.
Every Dublin subscriber was assigned a five-digit number, allowing calls within the city to be dialled directly without the help of an operator. However, calls to other towns and regions still required operator assistance, as long-distance connections remained manual for some years to come.
As technology advanced, the next goal was to extend direct dialling beyond city limits. In the 1950s and 1960s, Subscriber Trunk Dialling (STD) was introduced, allowing callers to make long-distance calls within Ireland without operator assistance by prefixing the local number with an area code. This development greatly reduced call-handling delays and marked another step towards a fully automated network. The international equivalent, International Subscriber Dialling (ISD), followed in the 1970s, enabling Irish subscribers to dial numbers abroad directly using country codes.
By the 1970s, Ireland’s telecommunications infrastructure was widely regarded as inadequate. Waiting times for new telephone lines could stretch up to two years, and the network was struggling to keep pace with both demand and international standards.
In response, the government commissioned a comprehensive review of the sector. The resulting Dargan Report, named after the chair of the review committee, made several decisive recommendations: telecommunications should be separated from postal services, modernised through the adoption of digital technology, and managed as a commercially oriented, semi-autonomous organisation rather than as a traditional government department.
When Albert Reynolds was appointed posts and telegraphs minister in 1979, he moved swiftly to implement these reforms. Determined to modernise Ireland’s outdated network, Reynolds oversaw a complete restructuring of the sector.
In 1984, the telecommunications division of the Department of Posts and Telegraphs was formally spun off into a new semi-state company, Telecom Éireann, tasked with operating on a self-funding, commercially sustainable basis.
To accelerate digitalisation and improve service levels, Telecom Éireann signed a significant bulk-supply agreement with a vendor consortium led by Ericsson and Alcatel. Both companies established manufacturing operations in Ireland – Ericsson in Athlone and Alcatel in Bandon – as part of the national modernisation drive. In 1982, Athlone became home to the country’s first fully digital local exchange, signalling the beginning of a technological transformation that would reshape Irish communications.
Computerised digital exchanges
The pace of progress during the 1980s was dramatic. Over the course of a few years, Ireland’s ageing electromechanical exchanges – manual, step-by-step, and crossbar – were systematically replaced by computerised digital exchanges using time-division multiplexing and common-channel signalling.
When Telecom Éireann formally assumed control in January 1984, only 309 manual exchanges remained out of roughly 1,000 nationwide. By 1987, the last of these legacy switchboards had been retired, marking the end of operator-assisted calling and completing the automation of the entire network. This was a remarkable achievement: by the late 1980s, Ireland had attained a 100% digital Public Switched Telephone Network (PSTN) – one of the first countries in the world to do so.
By 1991, Telecom Éireann had signed up its one millionth customer, a milestone that reflected both the success of its modernisation programme and the rising demand for reliable telephone service.
Another big development of the 1980s was the introduction of mobile telephony. In 1986, Telecom Éireann launched Eircell, Ireland’s first cellular network, based on the analogue TACS (Total Access Communication System) standard. Initially marketed as a car-phone-style service for business users in urban areas, coverage expanded rapidly, and by the early 1990s the analogue network reached more than 90% of the population.
Although mobile users were still a small minority compared with fixed-line subscribers, this new form of wireless communication represented a fundamental shift – it untethered the telephone from the wall socket and gave users unprecedented mobility. .
The fall
It was simple in the past. There was a telephone network and a phone. A phone that stayed in the hallway. This was the situation in the early 1990s when circuit-switched voice reached its zenith, representing the peak of traditional telephony. Shortly thereafter, the decline began – a transition illustrated in the chart.
Figure 2‑1: Global voice traffic.
There are two solid lines representing circuit switched voice. Black represents fixed voice which starts at 100% in 1990 and decays to zero by 2025, as the network is retired. Red represents mobile, which starts at zero in 1990, peaks in 2020, and then declines as more mobile traffic shifts to VoLTE, under the control of the IMS.
While it is true to say that operators have evolved mobile voice into VoLTE and VoWiFi, these services struggle to differentiate themselves from OTT voice applications, which are often free or bundled with other features. There are two broken lines represent packet-switched voice.
Black represents operator-hosted IMS traffic, which continues to grow as mobile circuit-switched traffic is migrated to the IMS, where they become VoLTE subscribers. The red broken line is non-operator traffic, primarily Skype voice traffic from 2005 and smartphone messaging clients from 2010
(A) 1990s – MOBILE EMERGENCE: The late 20th century saw the introduction of mobile phones, but their impact on total voice traffic was small initially. PSTN remained dominant through the early 1990s. By the mid-1990s, mobile cellular networks began capturing a share of voice minutes as the number of mobile subscribers grew. Still, in 1990-1995, landline PSTN carried almost all voice traffic globally. The liberalisation of the telecom industry in the 1990s set the stage for cheaper calls and increased competition.
(B) 2000s – MOBILE OVERTAKES FIXED: The early 2000s brought an explosion in mobile phone adoption. Affordable prepaid mobile plans and liberalised markets allowed billions of people (especially in developing countries) to get phones. Mobile circuit-switched voice traffic surpassed PSTN in many countries (eg, Scandinavia) by the mid-2000s. Meanwhile, voice over IP (VoIP) began to emerge: services like Skype (launched in 2003) started to carry meaningful traffic. Nonetheless, traditional circuit-switched voice (mobile and PSTN) still accounted for the largest slice of global voice.
(C) 2010s – OTT & IMS ERA: The 2010s saw the rapid growth of smartphones and IP-based communications. Traditional PSTN fixed-line usage continued to drop (many users 'cut the cord' in favour of mobile-only services. Mobile voice traffic continued to grow, but more importantly, it began transitioning from 2G/3G circuit networks to 4G IMS (IP Multimedia Subsystem). Voice over LTE (VoLTE) – an IMS-based voice service – began in about 2014, and by the late 2010s, it was widely deployed by carriers. At the same time, OTT voice services experienced big growth. This trend continued as smartphone messaging apps added voice and video calling features (eg, WhatsApp, Facebook Messenger, WeChat, Viber). Mobile social apps began eating into voice traffic.
(D) 2020s – ALL IP AND DECLINE OF PSTN: By 2020, PSTN’s share of voice traffic had dwindled to a few per cent (many countries are in the process of retiring PSTN networks entirely). The vast majority of fixed-line calls have migrated to VoIP (often via IMS or other IP platforms by telecom operators), and mobile calls are increasingly over 4G/5G IP networks. IMS-based voice (VoLTE and VoNR over 5G) has become the dominant technology for operators. Traditional circuit-switched mobile voice is shrinking as 2G and 3G networks are sunset; it is expected to decline to ~20% of traffic by 2025 and retirement in 2030.
Over-the-top (OTT) voice and messaging platforms such as WhatsApp, FaceTime, and Zoom now account for a substantial and growing share of human conversation. Increasingly, communication no longer depends on the physical circuits or dedicated exchanges that once defined the telephone age. Instead, it flows across packet-switched networks, carried as data by applications that connect people directly, instantly, and globally.
Mobile circuit-switched voice still lingers at the edges of the network – sustained by legacy systems and areas of limited IP coverage – but its days are numbered. By 2030, or shortly thereafter, circuit-switched transmission, signalling systems like SS7, and the great mechanical and digital switching matrices that powered the 20th century will fall silent. Their work complete, they will pass into history – superseded by the all-IP world, where communication has become not a service provided by the network, but an experience shaped by the software that rides upon it.
Coda
And so, the era of circuit switching quietly gave way to the age of packets. For more than a century, the circuit-switched telephone network formed the backbone of global communication. Its design, based on dedicated end-to-end connections, defined how the world spoke, organised, and conducted business. From magneto powered outposts to vast digital exchanges, each generation represented a remarkable engineering milestone that enabled billions of conversations.
Its gradual replacement by packet-switched and IP-based systems marks not a loss, but a natural transition. The principles of reliability, quality, and universality established during the circuit-switched era became the foundation for today’s fully digital networks. Though the PSTN has reached the end of its operational life, its legacy endures in every modern communication system – a reminder that each new technology stands upon the achievements of those before it.
Author: Brendan Kearns. At 17, Kearns joined the Air Corps and trained in Avionics at Baldonnel before moving into telecommunications. He earned a BEng in 1991 and joined Telecom Éireann as an executive engineer in the same year. In 2007, he completed an MEng in Telecommunications. After a long and varied career, Kearns left eir in January 2024 to work as an independent consultant and to write and publish his book, 'Evolution of Telecommunications'.