SIP & digital voice migration
Move voice to a SIP trunk, hosted SIP phone or another compatible IP phone system, sized for the concurrent calls your business needs.
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PSTN & ISDN REPLACEMENT · UK-WIDE
Replace old phone lines before 31 January 2027
Move business voice and legacy line-dependent equipment away from the traditional PSTN. Keep eligible numbers, choose the right SIP or hosted-voice route, and plan gateways, connectivity and resilience around the equipment you actually use.
Safety-critical equipment: alarms, lifts, telecare and other monitored services must be checked with the equipment manufacturer, maintainer or monitoring provider before migration.
ONE MIGRATION PLAN
What the UKDDI PSTN Replacement Toolkit can cover
Start with what each existing line actually does, then choose the replacement technology and test it before the legacy service is ceased.
Keep and port existing numbers
Plan the number port without cancelling the old service first, and identify any broadband or equipment dependencies before migration.
Number porting →Analogue adapters & gateways
Where suitable, retain analogue handsets or legacy PBXs using ATAs, FXS gateways or SIP-to-ISDN gateways selected for the application.
4G/5G & resilient connectivity
Use fixed or cellular IP connectivity where conventional broadband is unavailable, or add a second path where continuity is important.
Special-service line review
Identify alarms, lift phones, CCTV, payment terminals, door entry, fax, modems and monitoring devices before the old line is removed.
Multi-site migration planning
Standardise equipment, numbering and migration steps across branches while retaining a per-site test, acceptance and rollback plan.
SINGLE PSTN LINE REPLACEMENT
A complete replacement package without a new fixed broadband circuit
Designed for SMEs and single-line applications where installing SOGEA purely to replace one PSTN service would be excessive. The package combines an analogue-capable router, managed SIP voice and a roaming mobile-data SIM in one Device-as-a-Service package.
From£19.95/month + VAT36-month minimum term
New UK geographic number included. Existing eligible numbers can be ported for £19.95 + VAT. Proactive monitoring is available for £4.95/month + VAT. Coverage, device compatibility and emergency-use requirements must be checked before migration.
Small office or reception line
Keep a familiar analogue handset while moving the underlying service to digital voice over mobile connectivity.
Alarm, lift, kiosk or personal-care line
Potentially suitable where the equipment provider approves IP/cellular migration. Safety-critical and monitored equipment must be checked before order.
Where SOGEA is disproportionate
Use managed mobile connectivity for a single voice service instead of installing a fixed broadband product solely to retain one line.
FREE PLANNING TOOL
What should replace your old line?
Answer five questions. The checker will build a practical starting plan you can copy into a UKDDI enquiry. It is a planning aid, not a substitute for manufacturer approval on specialist equipment.
AUDIT BEFORE YOU MIGRATE
Look beyond the telephone on the desk
Older PSTN and ISDN services are often hidden behind equipment that has worked quietly for years. A migration inventory should record the line number, site, connected device, maintainer, current use, required uptime and whether the number needs to be retained.
This is particularly important for equipment that may use tones, modems, out-of-band signalling, line voltage or monitoring arrangements that do not behave like a normal voice call.
Read the PSTN switch-off planning guide →Include these in your line audit
- Business voice and reception lines
- ISDN2 / ISDN30 PBX connections
- Intruder and fire alarms
- Lift and emergency phones
- Door entry and intercoms
- Payment / EPOS terminals
- Fax machines and analogue modems
- CCTV and remote monitoring
- Telemetry and other line-connected devices
MIGRATION METHOD
A controlled replacement in four stages
Audit
Find every number, line and connected device, including services that are not used for ordinary voice calls.
Design
Choose SIP, hosted voice, gateway, fixed or cellular connectivity and the resilience level appropriate to each service.
Test
Install and test the replacement first, including inbound/outbound calls, failover and connected-device operation.
Migrate
Port required numbers and cease the legacy service only after acceptance checks and supplier dependencies are confirmed.
FAQ
PSTN and ISDN replacement questions
When is the UK PSTN switch-off?
Openreach states that traditional phone lines will move to digital by 31 January 2027. Businesses should migrate in advance so voice and any equipment connected to legacy lines can be tested before the old service is ceased.
Can I keep my existing business phone number?
Eligible UK numbers can normally be ported. Do not cancel the existing service before a port has been accepted and the replacement service has been tested. UKDDI can review the number and proposed migration route before you make changes.
Will my alarm, lift, fax or payment terminal work over VoIP?
Not automatically. Legacy devices should be checked with the equipment manufacturer, maintainer or monitoring provider. Safety-critical and monitored equipment may require a specialist IP or cellular replacement rather than a standard analogue telephone adapter.
What happens to digital voice during a power cut?
Digital voice and IP equipment normally require local power. Where continuity is important, the replacement design should consider UPS or battery backup, resilient connectivity, mobile or cellular fallback and a tested business-continuity route.
Can UKDDI replace an ISDN connection to an existing PBX?
Some legacy PBXs can be retained using a suitable SIP-to-ISDN gateway, while others are better migrated directly to SIP or hosted VoIP. Compatibility, supportability and resilience should be checked before equipment is ordered.
Can I use 4G or 5G for a PSTN replacement?
Yes, where coverage and the application are suitable. A 4G or 5G data connection can provide primary or resilient IP connectivity for voice and other supported services, but signal quality, addressing, power and failover requirements should be checked at each site.
Is there a simple package for replacing one PSTN line?
Yes. UKDDI offers a Single-Line PSTN Replacement package using a router with built-in analogue telephone adapter, a managed SIP voice service and a multi-network mobile SIM. Suitability still depends on coverage and the connected equipment, particularly alarms, lift phones, telecare and other monitored devices.
NEXT STEP
Discuss a PSTN replacement
Start with the lines you still rely on today
Send UKDDI the current number, what the line is used for, the site postcode and any connected equipment. We can use that information to work through the replacement route with you.