ISDN to VoIP Migration: Step-by-Step Business Guide

ISDN has been the backbone of UK business telephony for decades, but with the PSTN switch-off approaching in January 2027, every ISDN circuit in the country will cease to work. If your business still uses ISDN2 or ISDN30, migrating to VoIP is no longer optional — it is essential.

This guide takes you through the migration process step by step, from initial audit to go-live, so you can move confidently and avoid costly last-minute disruption.

Why ISDN Is Being Switched Off

ISDN (Integrated Services Digital Network) runs on the same copper infrastructure as analogue phone lines. Openreach is retiring this entire copper network and replacing it with all-IP services. Key points:

• Openreach stopped selling new ISDN lines in September 2023.
• Existing ISDN circuits will stop working by January 2027.
• There are no extensions or exceptions — the copper is being physically decommissioned.
• Businesses on ISDN30 (used for multi-line PBX systems) and ISDN2 (used for smaller setups) are both affected.

What Replaces ISDN?

ISDN is replaced by VoIP (Voice over Internet Protocol), which carries voice calls over your broadband or data connection instead of dedicated copper circuits. The main replacement options are:

• Hosted VoIP: A cloud-based phone system where the provider manages everything. You get handsets or softphones and a monthly per-user fee. Ideal for businesses that want to retire their on-premises PBX.
• SIP trunking: Replaces ISDN circuits with virtual SIP channels that connect your existing PBX to the phone network via broadband. Ideal if your PBX is modern and SIP-compatible.
• Microsoft Teams with Direct Routing: Uses Teams as the phone system with SIP trunks providing PSTN connectivity. Ideal if your business is already invested in Microsoft 365.

Step 1: Audit Your Current ISDN Setup

Start by documenting everything connected to your ISDN circuits:

• Circuits: How many ISDN2 and ISDN30 circuits do you have? At which sites?
• Channels: How many simultaneous call channels do you use? Check peak usage, not just total capacity.
• DDI numbers: List all your direct dial-in numbers. These can usually be ported to your new provider.
• PBX: What PBX system are you running? Note the make, model, and age. Check whether it supports SIP trunking.
• Connected services: Identify any non-voice services on the ISDN — conference bridges, EPOS, fax, alarm diallers.
• Contract dates: Check your current ISDN contract end dates to avoid early termination fees.

Step 2: Assess Your Internet Connectivity

VoIP relies on your internet connection for call quality. For each site, check:

• Bandwidth: Each simultaneous VoIP call uses approximately 100 Kbps. A 20-channel equivalent needs at least 2 Mbps of dedicated bandwidth — but aim higher to accommodate data traffic too.
• Latency and jitter: Latency should be under 150ms and jitter under 30ms for good call quality. Run a VoIP-specific speed test.
• Connection type: FTTP (fibre to the premises) is ideal. FTTC with SoGEA is adequate for most small and medium sites. ADSL is not suitable for VoIP.
• QoS: Ensure your router or firewall supports QoS to prioritise voice packets over general data traffic.

If your broadband is not up to the job, upgrade it before migrating your phone system. This is the single most common cause of poor VoIP quality.

Step 3: Choose Your VoIP Solution

Based on your audit, select the right approach:

• If your PBX is modern and SIP-compatible: SIP trunking is usually the quickest and cheapest migration path. You keep your PBX and replace the ISDN circuits with SIP trunks.
• If your PBX is old or end-of-life: Hosted VoIP replaces both the PBX and the ISDN in one move. You get a new cloud phone system with no on-site hardware to maintain.
• If you use Microsoft 365: Teams with Direct Routing consolidates your phone system into Teams, eliminating the need for a separate PBX altogether.

Step 4: Select a Provider

When evaluating VoIP providers, consider:

• UK-based support with engineers who understand your setup
• Number porting experience — ask how long porting typically takes
• SLA commitments on uptime and call quality
• Inclusive call bundles vs. pay-per-minute pricing
• Hardware options (desk phones, DECT handsets, headsets)
• Integration with your CRM, helpdesk, or Microsoft 365
• Managed migration service — will they handle the project end to end?

Step 5: Port Your Numbers

Number porting moves your existing phone numbers from the old ISDN provider to your new VoIP provider. Key details:

• Geographic numbers (01/02) and non-geographic numbers (03, 0800) can almost always be ported.
• Porting typically takes 10–20 business days, depending on the losing provider.
• During the porting window, calls are redirected seamlessly — there should be no downtime.
• Start porting early. Last-minute porting requests in 2026–2027 may face longer queues.

Step 6: Configure and Test

Before going live:

1. Configure your new VoIP system — extensions, call groups, auto attendants, voicemail, and call recording.
2. Set up handsets or softphones for each user.
3. Run a two-week pilot with a small group. Test internal calls, external calls, call quality, and features.
4. Gather feedback and adjust configuration as needed.

Step 7: Go Live and Decommission ISDN

1. Once the pilot is successful, migrate all users to the new system.
2. Confirm number porting is complete and all calls route correctly.
3. Contact your ISDN provider to cancel circuits and avoid ongoing charges.
4. Remove or repurpose old PBX hardware.
5. Brief your team on the new system and provide quick-reference guides.

For a detailed look at what to expect during a managed VoIP migration, see our article on managed VoIP migration. If SIP trunking is your chosen path, our guide to SIP trunking explained covers costs and technical details.