PSTN Transformation

by Paul Beaumont

In this timely and technically rich article, Paul Beaumont—former Head of Fixed Connectivity at

the UK Ministry of Defence—offers a comprehensive overview of the transformation of the Public Switched Telephone Network (PSTN) as it approaches its final shift to Voice Over Internet Protocol (VOIP) by January 2027. With a focus on the implications for Defence, Paul traces the historical evolution of telephony infrastructure, outlines the drivers behind the transition, and details how the MOD is navigating the complex migration from legacy services. This piece is essential reading for professionals engaged in communications strategy, infrastructure planning, and digital transformation across government and critical national infrastructure. Paul offers his personal perspective and understanding of the challenges and approach being taken to transformation having recently left a senior leadership role Defence Digital.

App Summary

We are on the cusp of the finalisation of the latest transformation of the Public Switched Telephone Network (PSTN) to Voice Over Internet Protocol (VOIP). Whilst it is the end of the PSTN as we have hitherto known it, nevertheless, talk of the “shutdown” and “closure” of the PSTN is incorrect. The ability to make/receive calls and speak to someone is very much still valued and hence the PSTN will continue. It’s just that the underlying technology, on which it is based, has over the last two decades been progressively shifting from being Time Division Multiplex (TDM) to Internet Protocol (IP) and at 31 January 2027 this transition will conclude. After which the TDM infrastructure that provides switching, transmission and synchronisation will be decommissioned. Thankfully the MOD is well advanced in removing its dependency on legacy TDM based services. Through either: (1) ceasing lines that are no longer required, (2) migrating to a strategic replacement service where there is an enduring need like Microsoft Teams certified VOIP phone physical handset and (3) remainder migrating to the tactical replacement service of Pre-Digital Phone Line (PDPL) to buy time in which to finish transition.

Link to full article.