The real deadline of ISDN and PSTN migration

Many IT leaders know that the UK's Public Switched Telephone Network (PSTN) and Integrated Services Digital Network (ISDN) services will be withdrawn on 31 January 2027. What is less widely understood is that the deadline that matters most is much sooner.

If your organisation still relies on ISDN or PSTN services, you should be planning your migration now and submitting migration orders before 30 September 2026.

That date is the industry's critical deadline for migration orders to help ensure changes can be completed ahead of the final switch-off. For organisations that have not yet started reviewing their estate, this is no longer just a telecoms project. It's a business continuity issue.

The deadline that matters most

The January 2027 switch-off date is fixed, but waiting until then is not an option.

According to guidance from Ofcom and BT Business, organisations still using legacy telephony and connectivity services should be acting now. The key deadline is 30 September 2026, the point by which migration orders should be submitted to maximise the likelihood of being transitioned before the final withdrawal of services.

For many businesses, the remaining challenge is not awareness. It's understanding what still relies on legacy infrastructure and giving themselves enough time to replace it.

What happens if you don't migrate?

The PSTN switch-off is not simply a technology upgrade. Once legacy services are withdrawn, organisations that have not migrated risk losing access to services that depend on those connections.

That could include:

• Business phone systems
• Fax services
• Lift emergency lines
• Alarm and security systems
• Payment terminals
• Other operational technologies connected to the PSTN network

This is where people get caught out.

Many organisations know they have a phone system to replace. Fewer know exactly what else is connected to legacy lines across their estate. The risk is often not the phone line itself. It's the critical service that nobody realised depended on it.

Why waiting creates risk

The closer organisations get to the switch-off, the less flexibility they have to identify dependencies, plan migrations and implement replacement services in a controlled way.

Delays can increase the risk of:

• Disruption to business-critical services
• Additional pressure on internal IT teams
• Discovering hidden legacy systems late in the process
• Paying more to maintain ageing infrastructure
• Rushed migration decisions

As we explored in our previous articles on the cost of delaying your PSTN migration and why connectivity is now a business continuity issue, organisations that leave changes until the last minute often have fewer options available to them.

A chance to modernise, not just migrate

While the immediate priority is avoiding disruption, the PSTN switch-off also creates an opportunity to review technology that may have been in place for years.

Many organisations are using the transition to:

• Consolidate ageing telephony platforms
• Remove redundant or unused services
• Improve resilience and business continuity
• Support hybrid and flexible working
• Modernise communications and connectivity infrastructure

Rather than simply replacing services like for like, now is a good time to assess whether your current setup still meets the needs of the business.

For IT leaders, the most successful approach is to treat PSTN migration as both a continuity exercise and a wider estate review.

What should happen next?

The key date to focus on is 30 September 2026, not just the final switch-off on 31 January 2027.

The organisations best positioned for the transition will be those that use the remaining time to do two things: identify and migrate services that still depend on PSTN or ISDN, and review their wider communications and connectivity estate for opportunities to modernise.

Don't delay. Speak to one of our specialists.