The clock is running. With the full UK PSTN and ISDN switch-off confirmed for 31 January 2027, every business still running traditional copper-based phone lines must migrate, or their phone lines won’t work. Stop-sell on new ISDN services has been in effect since September 2023, meaning most businesses can no longer order new lines or make amendments; existing lines are simply running down to the deadline.
For many IT decision-makers, the biggest concern isn’t whether to migrate, it’s how to do it without taking the phones offline. The fear of downtime is one of the main reasons businesses delay, and that delay is becoming increasingly risky. As the deadline approaches, demand for migration services will peak, installation queues will lengthen, and number porting slots will become scarce.
The good news: done properly, an ISDN migration to SIP trunks or hosted VoIP does not require any downtime. Here’s how it works.
Why the Deadline Is Closer Than It Looks
The January 2027 date is firm; Openreach has confirmed no further extensions will be granted. But the practical deadline for a comfortable migration is considerably earlier.
By the end of 2025, the majority of UK exchanges had reached Stop-Sell status. Existing ISDN lines continue to function for now, but they’re running on infrastructure that is no longer being invested in. Faults are taking longer to resolve. Support from providers is thinning. The closer you get to 2027, the more likely you are to experience degraded service quality before the switch-off even happens.
Factor in the time required for a thorough audit, system procurement, configuration, testing, number porting, and staff training, and a migration that feels like it has plenty of runway can quickly become urgent. For multi-site businesses or organisations with complex configurations, the safe window is now!
Step 1: Audit Everything That Touches ISDN
The first and most important step is a comprehensive audit of every service connected to your ISDN lines. This is where most businesses are caught out; it’s rarely just the desk phones.
Common ISDN-connected services that are frequently overlooked include:
- PBX systems: the most obvious, but the gateway to everything else
- DDI (Direct Dial-In) number ranges: every number your customers call
- Fax machines: still in use in legal, healthcare, and professional services
- Alarm systems: many intruder and fire alarms use ISDN for monitoring calls
- Lift phones and emergency lines: legally required in many buildings
- Payment terminals (PDQ machines): some older card readers use analogue or ISDN connections
- Door entry systems
- CCTV monitoring lines
Do not rely on your phone bill alone to identify lines. Physical line tracing is often necessary, and businesses regularly discover lines they didn’t know they had. Every one of these connections needs to be accounted for before migration begins, because each requires its own replacement solution.
Step 2: Assess Your Internet Connectivity
Unlike ISDN, which runs on a dedicated circuit, SIP trunks and hosted VoIP carry voice traffic over your internet connection. Your broadband or leased line therefore becomes the foundation of your phone system.
Before migrating, your connectivity needs to be tested and assessed against the demands of your call volumes:
Bandwidth
A single HD voice call requires approximately 85–100 Kbps. Calculate your peak concurrent call requirement and ensure sufficient headroom alongside your normal data usage.
Latency
Voice is sensitive to delay. Latency above 150ms in each direction begins to affect call quality perceptibly. Test under real-world load conditions.
Jitter
Variation in packet delivery time causes audio degradation. A well-configured connection should see jitter below 30ms.
Packet loss
Even 1–2% packet loss causes audible problems on voice calls. Any packet loss should be investigated and resolved before migration.
If your existing connectivity doesn’t comfortably support your voice requirements, this is the moment to upgrade. A fibre leased line with a guaranteed SLA is the gold standard for business VoIP, and in most cases, the combined cost of leased line plus SIP trunks is lower than what you’re paying for ISDN lines today.
Quality of Service (QoS)
The configuration on your router is also essential. QoS rules prioritise voice traffic over other data on your network, preventing a large file download or video stream from degrading active calls. Your provider or IT team should configure this as part of the migration.
Step 3: Choose Your Migration Path
There are three main approaches to ISDN replacement, and the right one depends on your existing infrastructure, budget, and appetite for change.
Option 1: SIP Trunks to an Existing PBX
If your business already has a PBX (whether on-premise or hosted) and wants to keep it, SIP trunks replace the ISDN connection that feeds it. Your PBX continues to manage extensions, call routing, and features, the only change is the connection to the outside world.
What this requires: Your PBX must be SIP-compatible. Most modern IP-PBX systems are. If your PBX is older and only has physical ISDN ports, a Session Border Controller (SBC) or media gateway can bridge the gap. It sits between your internet connection and your PBX, translating SIP into the ISDN signalling your PBX understands. This extends the usable life of legacy hardware without requiring a full replacement.
This is often the lowest-disruption option for businesses with an existing, working phone system.
Option 2: Hosted VoIP (Cloud PBX)
A hosted VoIP solution replaces both the ISDN lines and the on-premise PBX with a single cloud-based service. Your phones, desk phones, softphones, or mobile apps connect directly to the cloud platform, with no physical PBX hardware on site.
This is the most future-proof approach and typically delivers the richest feature set. Call recording, voicemail-to-email, auto attendants, ring groups, real-time analytics, and CRM integration are all available as standard. For businesses looking to modernise their communications infrastructure rather than simply replace copper with fibre, hosted VoIP is the right destination.
Option 3: Hybrid Migration
For larger or more complex businesses, a phased hybrid approach runs SIP trunks and ISDN lines in parallel for a period of 30 to 60 days. Calls can be routed via SIP for testing and validation while ISDN remains live as a fallback. This de-risks the transition and allows thorough testing without any pressure on the live environment.
This approach adds some cost but is the right choice for organisations where telephony downtime carries significant operational or reputational risk.
Step 4: Number Porting, The Critical Path
Your phone numbers are one of your most valuable business assets. Customers know them; they’re on your stationery, your website, your advertising. Losing them during migration, even temporarily, is not acceptable.
Number porting is the process of transferring your existing DDI numbers from your current ISDN provider to your new SIP or VoIP provider. It’s a regulated process in the UK, managed through Openreach, and there are important things to understand:
- Category A (simple) ports, single number or small range, same site, typically take around 10 working days
- Category C (complex) ports, multi-site, large DDI ranges, or numbers ported from complex configurations, can take 30 days or more
This means porting needs to be initiated early, not at the end of the migration process. Businesses that leave number porting to the last step frequently find themselves in a window where neither the old nor the new system is fully live.
The zero-downtime approach to number porting:
- Set up your new SIP trunks or hosted VoIP platform fully, with test numbers, before initiating the port
- Configure call forwarding from your existing ISDN numbers to the new system while the port is in progress
- Test all inbound and outbound call flows on the new platform with the temporary numbers
- Once porting completes, your existing numbers route directly to the new system, the forwarding is no longer needed
- Decommission the ISDN lines only after confirming all numbers are live on the new platform
At Fuse 2, our Orca Portal supports number porting across 240+ locations globally, with direct management of the porting process and real-time status updates. We handle the administrative process, Letters of Authority, technical validation, Openreach liaison, so your team doesn’t need to!
Step 5: Configure, Test, and Validate
Before any cutover, the new system must be tested exhaustively, not just that calls connect, but that every feature and call flow behaves exactly as expected.
A proper pre-cutover test plan includes:
Inbound testing
- Call each DDI number and confirm it routes correctly
- Test auto attendants and IVR menus
- Test call queues under load (multiple simultaneous inbound calls)
- Confirm voicemail and call recording are working
- Verify out-of-hours routing
Outbound testing
- Confirm outbound calls connect and the correct caller ID is presented
- Test international dialling if required
- Verify emergency services (999) routing, this is a legal requirement
- Test calls to mobiles, geographic numbers, and non-geographic numbers
Load testing
- Make the maximum number of simultaneous calls your system should support
- Listen for clipping, echo, one-way audio, or quality degradation under load
Failover testing
- Simulate an internet connection failure and confirm calls divert as configured
- Test the disaster recovery routing (typically to mobile numbers or an alternative site)
One-way audio, where one party can hear the other but not vice versa, is one of the most common issues encountered during ISDN-to-SIP migration. It’s almost always caused by SIP ALG (Application Layer Gateway) being enabled on the router or firewall. SIP ALG interferes with the SIP signalling and should be disabled on all network equipment before go-live.
Cutover should always take place outside business hours, typically in the late evening or on a weekend. Even with a parallel run in place, the moment of final cutover carries some risk, and doing it when call volumes are at zero means any last-minute issues can be resolved without customer impact.
Step 6: Non-Phone ISDN Devices
This is the step businesses most commonly overlook until it’s too late. Alarms, lifts, payment terminals, and other non-phone devices that use ISDN or analogue connections need their own replacement plan, and some of these require lead times that rival or exceed the phone migration itself.
Alarm systems
Most modern alarm panels support IP-based communication as standard or via an IP communicator add-on. Contact your alarm monitoring provider to confirm the path and get it scheduled early, installation slots fill up.
Lift phones
Building regulations require functional emergency communication in lifts. Traditional lift phones using ISDN can be replaced with GSM/4G units or IP-based alternatives. This typically involves a maintenance visit and should be coordinated with your lift service provider.
Payment terminals
Most modern PDQ machines already use IP or 4G connectivity. If yours connects via a phone line, contact your payment provider to arrange an upgrade. This is not a migration you can do yourself.
Fax
The simplest solution in most cases is a fax-to-email service, which receives faxes to your existing fax number and delivers them as PDF attachments to an email address. Analogue Telephone Adapters (ATAs) can also keep physical fax machines working over a VoIP connection where needed.
Common Migration Myths, Addressed
“Migration will mean downtime.” Not with a properly managed parallel deployment and phased cutover. Most businesses experience zero downtime when migration is handled by an experienced provider.
“SIP call quality is worse than ISDN.” SIP trunks on a well-configured network consistently deliver HD voice quality, noticeably better than ISDN in most cases. Quality issues during migration are almost always network configuration problems, not inherent to SIP.
“We’ll lose our phone numbers.” Number porting is a regulated process. Your numbers are yours to keep. With the right provider managing the porting process, your numbers transfer seamlessly.
“Our PBX won’t work with SIP.” Most modern PBX systems are SIP-compatible. Older systems can be connected via a media gateway or SBC, extending their life considerably. Only very old TDM-only hardware without any IP capability would need replacing.
“We can leave it until next year.” With a hard deadline of January 2027, a stop-sell already in effect, and demand for migration services expected to peak through 2026, leaving migration until the final months carries real risk, rushed decisions, limited installer availability, and higher costs.
The Fuse 2 Migration Approach
Fuse 2 specialises in managed migrations from ISDN to SIP trunks and hosted VoIP, built on our own carrier-grade global network. We handle the process end-to-end, audit, connectivity assessment, system design, number porting, configuration, testing, and cutover, so your team doesn’t need to manage the complexity.
Our Orca Portal gives you full visibility of the porting process in real time, with instant provisioning of SIP trunks, DID numbers, and 3CX licences across 240+ locations. And because we own the infrastructure, we can provide the speed and accountability that a reseller simply can’t.
If your business is still on ISDN and you haven’t started planning your migration, now is the right time. The comfortable window is narrowing.
Book a free ISDN migration consultation with the Fuse 2 team →
