Andrew Mercer
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Voice over IP

The traditional phone network is circuit switched: a call reserves a dedicated 64 kbps path end to end for its whole duration. The internet is packet switched: data is split into packets that share links with everything else. Once internet bandwidth became cheap and plentiful, carrying voice as packets made economic sense.

  • 1995: VocalTec released InternetPhone, one of the first consumer VoIP programs.
  • 1996 onward: the ITU's H.323 standard defined early VoIP signalling.
  • 2002: the Session Initiation Protocol (SIP), RFC 3261, was standardized. It became the dominant VoIP signalling protocol. SIP sets up calls, and the audio itself travels separately over RTP.
  • 2002–2004: consumer VoIP services such as Vonage and peer-to-peer Skype (2003) brought cheap and free calling to ordinary users.
  • 2000s–2010s: cable companies and then traditional carriers moved their own "home phone" services onto VoIP. To customers it often looked the same as a regular landline.

An analog telephone adapter (ATA) connects an ordinary phone to a VoIP service. The ATA presents an FXS port, which mimics a central office line by supplying loop current, dial tone, and ring voltage, and it handles SIP on the network side. Carrier modems and fibre terminals usually have an ATA built in. For running VoIP yourself, with SIP providers and PBX software, see the VoIP section.

Copper retirement

Copper loops are expensive to maintain. They corrode, get wet, get damaged by construction, and are increasingly stolen for their metal value. Carriers around the world are replacing them with fibre and moving voice service to VoIP over that fibre:

  • In the United States, the FCC streamlined its copper retirement rules in March 2026, and carriers have been speeding up the shutdown of copper plain-old-telephone service since then.
  • In Canada, in a 2026 CRTC proceeding, Bell Canada described its copper facilities as frequently trouble-prone and said it was making ongoing efforts to replace them with new fibre.
  • In Toronto's Swansea neighbourhood, Bell notified customers that copper home phone service would shut down on November 4, 2026 unless they moved to fibre.

What changes for users

  • Power outages: a copper line is powered from the central office. A fibre phone depends on a powered modem in the home, so it goes dark during a blackout unless a backup battery is installed. In 2026 Bell was selling a battery pack offering up to nine hours of backup.
  • Legacy devices: medical alert systems, alarm panels, fax machines, elevator phones, and dial-up modems were designed for analog copper. Some do not work reliably over VoIP. Medical alert provider Connect America reported that nearly 40% of its Canadian customers still use devices that plug into a copper landline jack, and raised concerns about getting little advance notice of shutdowns.
  • Rotary phones: pulse dialing was designed around the central office detecting interruptions in DC loop current. Whether a VoIP adapter still recognizes those interruptions depends entirely on the device (see Using a rotary phone today).

The end of the PSTN

The public switched telephone network as a separate, circuit-switched system is being phased out around the world. Many countries in Europe and Asia have already completed or scheduled their migration to all-IP networks. Phone numbers, 911, and the experience of picking up a handset and hearing dial tone continue, but they are increasingly simulated by software running on packet networks.

Sources and further reading