The PSTN (Public Switched Telephone Network) is the aggregate of the world’s circuit-switched telephone networks — the traditional infrastructure of copper wire, fibre, switching centres, and mobile networks that has connected telephone calls since the 19th century. When a VoIP call reaches the PSTN, a voice termination carrier handles the interconnection between IP infrastructure and the legacy telephone network. Despite the growth of IP communications, the PSTN remains the final delivery mechanism for calls to standard phone numbers globally.
The PSTN (Public Switched Telephone Network) is the global network of copper wire, fibre, microwave links, and switching infrastructure that has carried telephone calls since the late 19th century. Despite the rise of VoIP and mobile communications, the PSTN remains active and essential — billions of calls terminate on PSTN-connected numbers every day. However, major telecoms regulators and carriers worldwide are phasing out the legacy circuit-switched infrastructure in favour of all-IP networks; the UK’s PSTN switch-off, for example, is scheduled for 2027.
PSTN uses circuit-switched technology — a dedicated physical circuit is established between caller and recipient for the duration of the call, using analogue or digital signals over physical copper or fibre infrastructure. VoIP uses packet-switched technology — voice is converted into data packets and transmitted over shared IP networks alongside other internet traffic. PSTN infrastructure is expensive to build and maintain but delivers highly reliable, low-latency calls. VoIP is more flexible, scalable, and cost-effective, but call quality depends on network conditions. Most modern calls start and end as VoIP and only touch PSTN infrastructure at legacy endpoints.
When a VoIP call needs to reach a standard PSTN phone number, it passes through a voice termination carrier that operates the interconnection between IP and PSTN networks. The call travels over the internet as SIP/RTP packets to the termination carrier’s switch, which converts it from IP to a circuit-switched signal (or routes it over an interconnect trunk) and delivers it to the destination network. IDT Express operates this interconnection, maintaining direct trunks with hundreds of PSTN and mobile carriers worldwide to deliver calls to virtually any phone number globally.
PSTN switch-off (also called PSTN replacement or the all-IP migration) refers to national programmes by telecoms regulators and incumbent carriers to retire legacy circuit-switched PSTN infrastructure and replace it with all-IP networks. In the UK, BT Openreach has announced a switch-off by December 2027; in Germany, Deutsche Telekom has been migrating customers to IP since 2018; in the US, the FCC has approved applications for carriers to retire copper PSTN infrastructure in specific areas. For businesses, the switch-off means all voice services must be migrated to VoIP/SIP-based solutions before PSTN lines are decommissioned.
