ISDN is being permanently switched off. The clock is ticking.
BT is discontinuing ISDN2 and ISDN30 as part of the national PSTN switch-off in January 2027. Every UK business still running ISDN lines will lose their phone service overnight unless they migrate. Beyond the switch-off, ISDN is already overpriced — SIP trunking delivers the same or better call quality at an average of 40–50% lower monthly cost.
Why Centric Comms
Centric Comms SIP trunks are delivered over resilient BT wholesale infrastructure, with per-channel pricing, instant scalability and built-in fraud protection. You keep your existing PBX and phone numbers — we simply replace the lines underneath. Most migrations complete with zero downtime.
Products & Pricing
Choose the right solution
All products include UK-based support, static IP where applicable and a named account manager. No hidden costs.
ISDN2 Replacement
SIP Trunk Channels
Pay only for the concurrent call channels you need. Scale up or down monthly with no engineer visits. Each channel supports one simultaneous call — most businesses need 1 channel per 3–5 users.
✓From 2 channels up
✓Unlimited UK geographic calls
✓DDI numbers included
✓Instant channel scaling
ISDN30 Replacement
SIP Trunk Bundle
High-capacity bundles for businesses replacing ISDN30 or large multi-line systems. Includes a large DDI block, inclusive call minutes and priority routing for business hours.
✓20–100+ channels
✓DDI block 01/02/03 numbers
✓Inclusive call minutes
✓Disaster recovery routing
Add-On
Number Porting
Keep every existing business number through the migration. We port your geographic, 0800 and non-geographic numbers to the SIP platform with maintained service throughout.
✓All number types ported
✓Zero downtime migration
✓CLI presentation maintained
✓Same-day porting available
Common Concerns
Questions we hear every day
Switching supplier always raises questions. Here are honest answers to the ones we hear most often.
🖥️
PBX Compatibility
Most modern IP-capable PBX systems (Avaya, Mitel, 3CX, Yealink) are SIP compatible. We run a free compatibility check on your existing hardware before any contract is signed.
🔌
Broadband Dependency
SIP trunks run over your internet connection. We'll assess your current bandwidth and recommend whether an upgrade is needed to support your call volume.
🔒
Fraud Risk
VoIP fraud is a real risk. Our SIP trunks include real-time fraud monitoring, spend caps and automatic block rules to protect your business from unexpected charges.
SIP Trunks replace traditional ISDN or analogue phone lines with internet-based voice channels. They connect your existing on-site PBX to the public telephone network via your broadband or leased line.
Yes — SIP Trunks are designed to work with your existing PBX hardware. You keep your system and simply replace the ISDN lines with SIP channels.
As a rule of thumb, you need one channel per simultaneous call. A business making up to 10 calls at once needs 10 channels. We help you right-size this during our free assessment.
SIP Trunks are not affected — they already run over IP. If you currently use ISDN, switching to SIP Trunks now is the straightforward way to future-proof before the deadline.
Ready to get started?
Talk to the Centric Comms team today
No obligation. No jargon. Just an honest conversation about what your business needs and what it would cost.