Switching from ISDN to VoIP: A Complete Migration Guide

Why ISDN Is Being Switched Off

BT Openreach has confirmed that the PSTN and ISDN networks will be switched off by 2027. This means every business currently using ISDN lines — whether ISDN2 (BRI) or ISDN30 (PRI) — will need to migrate to an IP-based alternative. The PSTN switch-off is not optional; it is a fundamental change to the UK telecommunications infrastructure.

Openreach has already stopped selling new ISDN lines in many exchange areas, and the remaining exchanges will follow. If your business still relies on ISDN, now is the time to plan your migration to VoIP.

What Is ISDN vs VoIP?

Understanding the difference helps you appreciate why VoIP is the natural successor:

ISDN (Integrated Services Digital Network)

Digital phone lines delivered over the copper PSTN network
ISDN2 provides 2 channels (2 simultaneous calls) per line
ISDN30 provides 8–30 channels over a single connection
Requires an on-premise PBX to manage calls
Fixed to a physical location

VoIP (Voice over Internet Protocol)

Phone calls delivered over your broadband connection
No limit on simultaneous calls (depends on bandwidth)
Hosted in the cloud — no on-premise PBX needed
Works from any location with internet access
Includes advanced features as standard

Migration Planning Checklist

Before you begin your ISDN to VoIP migration, work through this checklist:

Audit your current setup — how many ISDN channels do you have? How many simultaneous calls do you typically make?
List all phone numbers — document every number you need to port, including DDI ranges
Check your broadband — ensure your internet connection can handle VoIP traffic (see our broadband requirements guide)
Identify connected equipment — fax machines, alarm systems, PDQ card terminals and lift phones may also use your ISDN lines
Review contracts — check termination dates and any early exit fees on your ISDN and PBX maintenance contracts
Set a timeline — allow 4–8 weeks for the full migration process

Step-by-Step Migration Process

Step 1: Assess and Plan

Work with your new VoIP provider to assess your current telephony setup, broadband capability and business requirements. Connection Technologies provides a free, no-obligation assessment for businesses migrating from ISDN.

Step 2: Choose Your VoIP Solution

Select the hosted platform, handsets and features that match your needs. Your provider will recommend the right configuration based on your call volumes, number of users and feature requirements.

Step 3: Configure the New System

Your VoIP system is configured in the cloud — auto attendant, ring groups, voicemail, call recording and all other features are set up before your numbers are ported.

Step 4: Port Your Numbers

Submit porting requests for all your existing numbers. The standard porting window is 5–10 working days. During this period, your ISDN service continues to work normally.

Step 5: Run Systems in Parallel

For larger businesses, we recommend a brief parallel running period where both your ISDN and VoIP systems are active. This allows you to test the new system thoroughly before fully committing.

Step 6: Go Live and Decommission ISDN

Once all numbers have ported and you are satisfied the new system is working perfectly, your old ISDN lines can be cancelled. Any on-premise PBX equipment can be decommissioned.

Cost Comparison: ISDN vs VoIP

The financial case for switching is compelling:

ISDN30 line rental: £300–400/month for 10–30 channels
PBX maintenance: £50–150/month
Call charges: £100–300/month (variable)
Total ISDN cost: £450–850/month

VoIP (equivalent users): £120–300/month (all-inclusive)
Typical saving: 40–65%

Common Concerns Addressed

Will call quality be as good?

With a suitable broadband connection, VoIP call quality is better than ISDN. VoIP supports HD wideband audio, while ISDN is limited to narrowband.

What about reliability?

Hosted VoIP platforms use redundant data centres with 99.99% uptime SLAs. Combined with a 4G backup connection, VoIP can be more resilient than ISDN.

What happens to our fax machines?

Traditional fax machines do not work reliably over VoIP. We recommend switching to an online fax service or fax-to-email solution, which is more efficient and cost-effective.

Can we keep our numbers?

Yes. All your existing numbers — geographic, non-geographic and DDI ranges — can be ported to your new VoIP system.

Ready to migrate from ISDN to VoIP? Get a free migration assessment from Connection Technologies and we will plan your transition step by step.