ISDN Migration Checklist: Everything You Need to Do Before January 2027

The deadline is 31 January 2027. After that date, Openreach switches off the PSTN and ISDN networks for good. Every business still running on ISDN lines will lose their phone service overnight.

That sounds far away. It isn't.

Engineers are already getting booked up. Providers are already managing migration queues. If you wait until late 2026, you will be scrambling — and you may not get the support you need in time.

This checklist walks you through every step of an ISDN migration. Work through it methodically and you'll arrive at switchover day without drama.

Step 1: Audit What You Have

Before you do anything else, find out exactly what's on your ISDN lines.

Count your channels. A standard ISDN2e gives you 2 channels. ISDN30 can carry up to 30. Write down how many simultaneous calls your business can currently make.

List every device connected to your PSTN/ISDN infrastructure. This is where most businesses get caught out. It's rarely just phones. Check for:

• Desk phones and reception handsets
• Fax machines
• Broadband (ADSL or FTTC delivered over the copper line)
• PDQ/card payment terminals
• Intruder alarms with a phone dialler
• Fire alarms with a phone dialler
• Lift emergency lines
• Door entry systems
• CCTV monitoring lines

Each of these needs its own plan. Some will migrate to VoIP easily. Others — alarms, lifts, PDQ machines — require a separate conversation with the relevant supplier.

Step 2: Check Your Broadband Readiness

VoIP runs over your internet connection. The quality of your calls depends on the quality of your broadband.

Test your current connection. You need:

• Latency below 150ms (ideally under 50ms)
• Jitter below 30ms
• Consistent upload speed — roughly 100 Kbps per concurrent call

Run a test at a tool like ping.canopy.tools or similar. Do it during your busiest call period, not at midnight.

If your broadband doesn't pass muster, upgrade it now. Don't migrate your phones and discover the problem on go-live day. Options include moving to FTTP (full fibre), an EFM leased line, or a dedicated SoGEA broadband line — the latter is designed specifically for businesses moving off PSTN.

Consider a separate voice VLAN. If you have a managed switch, segregating voice traffic from general data traffic prevents your video calls or file downloads from eating into call quality.

Step 3: Choose a VoIP Provider

This is the most important decision you'll make. Don't default to whoever sends the most marketing emails.

Ask these questions before committing:

• Are they a direct provider or a reseller? Resellers add a layer between you and the network. When something breaks, that layer slows everything down. A direct provider owns the infrastructure and can fix problems at source.
• Where is their support team based? Offshore support for a business-critical phone system is a risk.
• What does their SLA actually cover? Read it. "Best effort" is not an SLA.
• What are the contract terms? Long tie-ins are a red flag. Rolling 28-day contracts give you flexibility.
• Can they handle number porting? Not all providers do this themselves.

What to look for in plans: Most businesses need a mix of per-user minutes and features. Confirm whether call recording, auto-attendant, and mobile apps are included or charged as extras.

At VoIPninjas, we're a direct UK provider based in Christchurch, Dorset — not a reseller. Our plans run on 28-day rolling contracts with no lock-in:

• Ronin — £5.99/user/month — 100 UK minutes
• Samurai — £14.99/user/month — 750 UK minutes, call recording, auto-attendant, mobile app
• Shogun — £24.99/user/month — unlimited UK calls plus calls to 55 countries

Step 4: Decide on Hardware

You have three main options.

Softphones. An app on a PC, Mac, or smartphone. Zero hardware cost. Works well for staff who already work from a computer. The VoIPninjas mobile app is included on the Samurai plan upwards.

IP handsets. Desk phones designed for VoIP. Plug into your network switch via ethernet. Better call quality than a softphone in a noisy environment. Brands like Yealink and Grandstream are reliable and well-priced.

Analogue Telephone Adaptors (ATAs). If you have existing analogue handsets you want to keep using, an ATA converts the signal. It's the lowest-cost way to extend the life of older hardware. Quality varies — check compatibility with your provider first.

You don't have to pick one. Many businesses run a mix: IP handsets at reception, softphones for remote staff, and an ATA for a back-office analogue phone nobody wants to replace yet.

Step 5: Plan Your Number Porting

Number porting is the process of moving your existing phone numbers to your new VoIP provider. Your customers keep the same number. You just answer it differently.

List every number you want to keep. Geographic numbers, 0800 numbers, DDIs — all of it. Don't assume anything carries across automatically.

Start the porting process early. Standard porting takes 5–10 working days. Complex ports — particularly ISDN30 ranges — can take longer. Factor this into your timeline.

Don't cancel your ISDN line before porting completes. This is a common mistake. Cancelling the line before the port can make numbers irrecoverable. Port first, cancel after.

Step 6: Configure Call Routing Before Go-Live

Migrating to VoIP isn't just a like-for-like swap. It's an opportunity to set up your call routing properly.

Plan these before switchover day:

• Auto-attendant: What do callers hear? What options do you offer?
• Ring groups: Which phones ring for which incoming numbers?
• Voicemail: Who gets the message? Email or app notification?
• Out-of-hours rules: What happens at 5:30pm? On bank holidays?
• Call recording: Which extensions need this enabled?
• Hunt lists: In what order do phones ring if the first is busy?

Getting this right in advance means switchover day is a non-event. Getting it wrong means callers hit dead ends or the wrong person.

Step 7: Test Thoroughly Before Switching Off ISDN

Run both systems in parallel for at least a week. Keep your ISDN lines active while your VoIP system is live.

During the parallel run:

• Make and receive calls on the VoIP system
• Test from multiple locations — office, home, mobile
• Test during busy periods, not just quiet ones
• Check call recording is capturing correctly
• Confirm voicemail delivery works
• Test out-of-hours routing
• Have someone call in and work through the auto-attendant

Only when you're satisfied the VoIP system is working reliably should you proceed to the next step.

Step 8: Cancel Your ISDN Line

Once the migration is successful and your number porting is complete, notify your current provider to cancel the ISDN lines.

Check your notice period. Most ISDN contracts require 30–90 days' notice. Factor this into your timeline so you're not paying for both systems longer than necessary.

Keep a record of the cancellation confirmation. Billing disputes after cancellation are more common than they should be.

Step 9: Update Your Contact Details

If you ported your numbers, your phone numbers haven't changed. But it's still worth a quick audit:

• Website contact page
• Google Business Profile
• Email signatures
• Stationery and printed materials (if numbers changed)
• Directory listings

If you changed any numbers as part of the migration, this step becomes more important. Update everything before the old numbers stop working.

Step 10: Deal With the Edge Cases

Go back to the device list you made in Step 1. Every non-phone device connected to your PSTN line needs individual attention.

Alarms (intruder and fire): Contact your alarm company. Many modern panels support GSM/4G communication modules as an alternative to a phone line. This is now the standard approach.

Lift emergency lines: These are often a regulatory requirement. Contact your lift maintenance company. They need to sign off any change to the emergency communication method.

PDQ/card terminals: Contact your payment provider. Most modern terminals use broadband or 4G anyway. Confirm yours isn't still dialling out over a phone line.

Fax machines: The honest answer is to retire the fax machine. If you genuinely need to receive faxes, a fax-to-email service is the practical replacement.

Don't Leave This Until 2026

The January 2027 deadline will not move. Openreach has already extended it once. It won't extend again.

The closer you get to the deadline, the harder migration becomes. Providers will be overwhelmed with late-movers. Engineers will be booked weeks out. Number porting queues will lengthen.

The businesses that migrate smoothly are the ones that start now — while there's room to test, adjust, and fix problems at a sensible pace.

Ready to migrate before the rush? Start your free 14-day trial — no card required. We'll handle number porting and walk you through the migration. Get a Free Quote → or call us on 0330 043 2388 No tie-in, no setup fees. Most businesses are fully live within 10 working days.

Frequently Asked Questions

What happens to my phone numbers if I migrate to VoIP?

You keep them. Number porting moves your existing numbers to your new VoIP system. Your customers dial the same number and reach you as normal. The only thing that changes is the underlying technology delivering the call.

Can I migrate to VoIP if my broadband isn't very fast?

Speed matters less than stability. A 10 Mbps connection with low latency and low jitter will support VoIP calls better than a 100 Mbps connection that's inconsistent. Test your connection quality first. If it's poor, sort the broadband before you migrate the phones.

What if I have devices connected to my phone line that aren't phones?

This is common. Alarms, lifts, PDQ machines, and fax machines all need separate action. They can't simply be connected to a VoIP line the same way a phone can. Contact each relevant supplier individually and do this early — some lead times are longer than you'd expect.

How long does the migration take?

For most small businesses, 2–4 weeks from starting the process to being fully live. The parallel run period (where both ISDN and VoIP are active) is the part you shouldn't rush. VoIPninjas can have your account active within 10 working days of sign-up.