PSTN Switch-Off
BT deactivates all UK ISDN lines in January 2027 — migrate to SIP trunking before the deadline.
See migration guide →
United Kingdom
Carrier-grade SIP trunking for UK businesses
Ofcom CLI-compliant. ISDN replacement ready. UK geographic DID numbers, instant provisioning, and pay-as-you-go pricing in GBP.
- Ofcom CLI-compliant
- 160+ countries
- TLS / SRTP encryption
- No contracts
What is UK SIP trunking
SIP trunking UK: the direct answer
Definition
SIP trunking is a VoIP technology that replaces physical ISDN or PSTN phone lines with internet-based voice channels, connecting a UK business's PBX to the public telephone network over IP. With BT's PSTN switch-off scheduled for January 2027, SIP trunking has become the standard replacement for legacy ISDN2 and ISDN30 infrastructure across UK businesses.
IDT Express delivers carrier-grade SIP trunking built on 30+ years of wholesale voice infrastructure — the same network that Twilio, Genesys, and Microsoft Teams run on underneath. For UK businesses, that means Ofcom-compliant CLI handling, UK geographic DID numbers, direct carrier interconnect in London, and pay-as-you-go pricing with no channel minimums or long-term contracts.
BT PSTN switch-off
Your ISDN lines stop working in January 2027
BT is shutting down the UK's legacy Public Switched Telephone Network. Every business still running on ISDN2 or ISDN30 lines must migrate to an IP-based alternative before the deadline. SIP trunking is the direct, like-for-like replacement.
2023–2024
BT halts new ISDN line orders. Migration programme announced across all regions.
April 2025
Major migration push begins. Providers report accelerating transitions away from legacy lines.
Now
Migration window is open. Providers deploying at scale — act before capacity tightens.
January 2027 ⚠
Full ISDN deactivation. Any business still on legacy lines loses phone service.
ISDN to SIP migration guide
Most UK businesses complete this migration in 1–5 business days. Follow these steps in order — running both systems in parallel through Step 5 protects you against any disruption.
1
Audit your existing ISDN setup
Document what you have: how many ISDN2 or ISDN30 lines, how many channels (ISDN2 = 2 channels per circuit, ISDN30 = up to 30), and which DDI/DID numbers are in use. List every number used for inbound calls — these are the numbers you'll port. Check your BT contract for early termination clauses and note the earliest cancellation date.
✓ ISDN circuits & total channels
✓ Full list of DDI numbers
✓ BT/provider contract end date
✓ Peak concurrent call volume
2
Check your PBX for SIP compatibility
The majority of modern PBX systems are already SIP-compatible — no hardware upgrade required. Check your PBX vendor's documentation or firmware version. If your PBX was installed in the last 10 years, it almost certainly supports SIP trunking natively.
✓ SIP-ready (no changes)FreePBX, Asterisk, 3CX, Cisco CUCM 10+, Avaya IP Office, Mitel MiVoice, Microsoft Teams Phone
⚠ May need firmware updateOlder Cisco CME, legacy NEC, Panasonic KX-NS series — check vendor support pages
✗ Requires SIP gatewayAnalogue/TDM-only PBX — add an ATA (Analogue Telephone Adaptor) or SBC
3
Create your IDT Express account and test UK routes
Sign up for a free IDT Express account — you get £25 credit to run test calls before porting any numbers. Provision a temporary UK DID number, configure your PBX with the IDT Express SIP credentials, and verify call quality in both directions. Check that your CLI presents correctly on outbound calls to a UK mobile. This is your quality gate before you touch any live numbers.
4
Submit your DDI number port requests
Once satisfied with call quality, initiate porting of your existing UK DDI numbers to IDT Express. The process is paperless — you'll need your current provider's account details and the numbers to port. Under Ofcom GC18, your current provider must complete the port within one business day of the agreed date. Standard ports take 3–5 business days from submission.
✓ Current provider account number
✓ DDI numbers in +44 format
✓ Site address for the numbers
✓ Agreed porting date
5
Run both systems in parallel through cutover
Keep your ISDN lines active during porting. On porting day, your DDI numbers transfer to IDT Express and start routing over SIP. Monitor call quality, check inbound routing, and confirm outbound CLI presents your correct UK number. Run in parallel for 24–48 hours before cancelling ISDN circuits — this gives you a rollback option if anything unexpected comes up.
6
Cancel your ISDN lines with BT
Once you've confirmed stable operation on SIP for 24–48 hours, give BT (or your current ISDN provider) notice to cancel. Check your contract — most ISDN contracts require 30–90 days' notice. Submit cancellation in writing and keep confirmation for your records. You're now fully migrated and PSTN switch-off ready.
What's included
UK SIP trunking — feature overview
Everything you need for a production-grade UK voice deployment, included as standard.
| Feature |
IDT Express UK SIP |
Notes |
| UK Geographic DID Numbers | ✓ Included | 01/02 numbers — London, Manchester, Birmingham, Edinburgh + 60 UK area codes |
| Ofcom CLI Compliance | ✓ Carrier-level | Valid UK CLI presented on all outbound calls per GC17. Jan 2025 spoofing rules handled automatically. |
| UK Number Porting (DDI) | ✓ Paperless | Ofcom GC18 — ports complete in 1–5 business days, zero downtime |
| Concurrent Channels | ✓ Unlimited | No channel cap. Scale from 1 to 10,000+ concurrent calls on demand. |
| TLS / SRTP Encryption | ✓ Standard | Signalling and media encrypted end-to-end on all UK routes |
| UK Points of Presence | ✓ London PoP | Direct carrier interconnect in London for sub-50ms latency to UK PSTN |
| PSTN Switch-off Ready | ✓ Yes | Full IP infrastructure — no dependency on legacy BT circuits |
| Emergency Services (999) | ✓ Supported | 999/112 access supported. Location information guidance available. |
| MS Teams Direct Routing | ✓ Compatible | Works with all major SBCs: AudioCodes, Ribbon, Oracle, Cisco |
| Non-Geographic Numbers (03) | ✓ Available | 0300, 0330, 0345 — UK-charged non-geographic number ranges |
| Billing Currency | ✓ GBP | Pay-as-you-go, per-second billing in British Pounds |
| Contract | ✓ None | No minimum term, no setup fees, no channel minimums |
Regulatory compliance
Ofcom-compliant at the carrier level
UK voice regulation sits on top of every SIP deployment. IDT Express handles compliance at the carrier layer — so your calls are correctly authenticated, presented, and routed without your team having to manage it.
OFCOM GC17
CLI accuracy
All outbound calls present a valid, dialable UK number as the Calling Line Identity. No invalid or spoofed CLIs leave the IDT Express network.
JANUARY 2025 RULES
Anti-spoofing compliance
Ofcom now blocks international calls presenting UK numbers unless routed through a UK carrier. IDT Express handles UK CLI authentication at the network level — your calls get through, scammers don't.
OFCOM GC18
Number portability
UK customers can port their existing 01, 02, 03, and 0800 numbers to IDT Express. Paperless porting completes in 1–5 business days with zero service interruption.
OFCOM GC1
Emergency call access
999 and 112 emergency call access is supported on IDT Express UK SIP trunks. Location information guidance is available for business deployments.
Businesses using SIP trunking as end-users do not require Ofcom registration — your SIP trunk provider holds carrier authorisation. See our full UK Telecom Compliance Guide →
BYOC — Bring Your Own Carrier
Connect any UK platform to IDT Express carrier infrastructure
Already running Microsoft Teams, Genesys, or a CCaaS platform in the UK? You don't need to rip it out. BYOC lets you connect any SIP-compatible platform to IDT Express wholesale routes — cutting the retail markup and gaining direct UK carrier interconnect.
Microsoft Teams Direct Routing
Genesys Cloud
Amazon Connect
Twilio — BYOC
3CX
FreePBX / Asterisk
Zoom Phone
Cisco CUCM
AI Voice Agents (Pipecat, Vapi, Bland)
See all integrations →
Pricing
Pay-as-you-go UK pricing in GBP
No channel minimums, no monthly commitments. Start with a £25 free trial — no credit card required.
UK Geographic DID
£0.75/ number / month
- 01/02 UK area codes
- 60+ UK cities covered
- Instant provisioning
- Paperless porting available
Most popular
UK Voice Termination
£0.004/ min (UK fixed)
- Per-second billing
- Direct carrier routing
- No concurrent channel cap
- CLI passthrough included
UK Mobile Termination
£0.009/ min (UK mobile)
- 07 mobile number routing
- High ASR on UK mobile
- Real-time rate portal
- Volume discounts available
Rates shown are indicative. Current rates available in the IDT Express rate portal. Volume pricing available for 100k+ minutes/month.
Get started
UK SIP trunking setup in 4 steps
Most UK deployments are live within one business day.
1
Create a free IDT Express account
Sign up at idtexpress.com — no credit card required. Your account includes £25 free credit to test UK routes and verify call quality before committing.
2
Provision UK DID numbers
Choose from 60+ UK area codes — London (020), Manchester (0161), Birmingham (0121), Edinburgh (0131), and more. Numbers activate instantly. Porting existing numbers? Submit a paperless port request — standard completion is 1–5 business days.
3
Configure your PBX or platform
Enter your IDT Express SIP trunk credentials into your PBX (FreePBX, 3CX, Asterisk, Cisco) or platform (Microsoft Teams Direct Routing, Genesys, Twilio BYOC). Configuration guides are available for all major systems.
4
Test and go live
Place a test call to verify UK CLI presentation and call quality. For ISDN migrations, run both systems in parallel until you've confirmed stable operation, then cancel your legacy ISDN lines with BT or your current provider.
FAQ
UK SIP trunking — common questions
What is SIP trunking and how does it work in the UK?+
SIP trunking is a VoIP technology that replaces traditional ISDN or PSTN phone lines with internet-based voice channels. In the UK, SIP trunks connect your PBX or phone system to the public telephone network over IP, allowing businesses to make and receive calls without physical copper lines. With BT's PSTN switch-off scheduled for January 2027, SIP trunking has become the standard replacement for legacy ISDN infrastructure across UK businesses.
What is replacing ISDN in the UK after the BT PSTN switch-off?+
SIP trunking is the direct replacement for ISDN lines after the BT PSTN switch-off. Businesses migrating from ISDN2 or ISDN30 can connect their existing on-premise PBX to a SIP trunk provider like IDT Express and keep their UK phone numbers through number porting. The migration is straightforward — no new hardware is required in most cases, as modern PBX systems are SIP-compatible. IDT Express offers UK SIP trunks with instant provisioning and paperless number porting.
Do UK businesses need Ofcom authorisation to use SIP trunking?+
No. UK businesses using SIP trunking as end-users do not require Ofcom authorisation. Ofcom's General Conditions of Entitlement apply to communications providers — companies supplying SIP services to others. As a business purchasing SIP trunking to power your own phone system, you are not classified as a communications provider and do not need to register with Ofcom. Your SIP trunk provider (such as IDT Express) holds the required carrier authorisation and handles compliance with Ofcom General Conditions.
What are the Ofcom CLI rules UK businesses need to follow in 2025?+
From January 2025, Ofcom requires that calls presenting a UK CLI originating internationally must be routed through a UK network before reaching the recipient, or they will be blocked. For UK businesses with international offices making outbound calls using UK numbers, calls must originate on-shore or be verified by a carrier capable of authenticating the UK origin. Ofcom General Condition GC17 also requires all outbound calls to present a valid, dialable UK number as the CLI. IDT Express handles CLI compliance at the carrier level, ensuring your outbound calls are correctly authenticated and presented.
Can I keep my existing UK phone numbers when switching to SIP trunking?+
Yes. UK number portability is governed by Ofcom General Condition GC18, which requires providers to complete number ports within one business day. When switching to IDT Express SIP trunking, you can port your existing UK geographic (01/02), non-geographic (03), or freephone (0800) numbers. The porting process is paperless and designed for zero downtime. Once ported, your numbers are hosted on IDT Express infrastructure and will continue to work through the BT PSTN switch-off.
What is the difference between UK DID numbers and SIP trunks?+
UK DID numbers (also called DDI numbers — Direct Dial In) are individual telephone numbers assigned to your business, such as a London 020 or Manchester 0161 number. SIP trunks are the voice channels — the capacity to make and receive concurrent calls. Think of DID numbers as addresses and SIP trunks as the lines. You can have one SIP trunk with multiple DID numbers pointing to it, or scale to hundreds of concurrent channels. IDT Express provides both: UK DID numbers and the carrier-grade SIP infrastructure to route calls through them.
How do I set up SIP trunking for Microsoft Teams in the UK?+
SIP trunking for Microsoft Teams in the UK is configured using Direct Routing — a Microsoft feature that connects Teams Phone to an external SIP trunk provider via a Session Border Controller (SBC). The setup involves: (1) configuring your SBC with IDT Express SIP trunk credentials, (2) creating a voice routing policy in Teams admin, (3) assigning UK phone numbers (DIDs) to users, and (4) routing outbound calls through the IDT Express UK interconnect. IDT Express is compatible with all major SBCs including AudioCodes, Ribbon, and Oracle. See our Teams Direct Routing guide for the full walkthrough.
Related resources
UK telecom — related pages
Everything you need for a complete UK voice deployment.
Start UK SIP trunking today
£25 free credit. No contract. Ofcom-compliant from day one.
