Replace your ISDN lines before BT forces your hand.
BT is withdrawing ISDN in 2027. SIP trunking is the direct replacement. Keep your existing phone system, port your numbers with zero downtime, and cut your line costs by up to 55%. UK-wide migration managed by IP Four.
2027
BT ISDN switch-off deadline. Every UK business must migrate before this date
48 hrs
typical provisioning time for SIP trunks once compatibility is confirmed
55%
average reduction in line costs when replacing ISDN with SIP trunking
UK-wide
ISDN replacement capability across all UK regions and carriers
Everything needed for a zero-downtime ISDN migration.
PBX Compatibility
SIP trunks connect directly to your existing PBX using a SIP gateway or built-in SIP support. No need to replace your phone system. Your handsets, call flows, and extensions all stay exactly as they are.
Number Porting
Your existing geographic and non-geographic numbers are ported to the SIP platform with zero downtime. Customers call the same numbers they always have. Nothing changes from their perspective.
Like-for-Like Channel Replacement
Each ISDN channel is replaced with a SIP channel on a one-to-one basis. Call capacity is maintained throughout the migration. No engaged tones, no dropped calls, no disruption to your team.
Broadband Delivery
SIP trunks are delivered over your existing broadband or leased line connection. No new physical infrastructure required. If your connection is already suitable, migration can happen within 48 hours.
Codec and Quality Configuration
Call quality configured to match or exceed your current ISDN experience. G.711 and G.729 codecs supported. QoS settings applied to prioritise voice traffic over your network.
Migration Project Management
Dedicated migration manager for every ISDN replacement project. Cutover scheduled outside business hours. Full rollback plan in place. Nothing left to chance on go-live day.
From ISDN audit to SIP go-live in days.
ISDN Line Audit
We review your current ISDN configuration, number of channels, DDI ranges, and call volumes. Identify exactly what needs to be replicated on the SIP platform before any migration begins.
SIP Compatibility Check
Your existing PBX is checked for SIP compatibility. If a gateway is required, we specify and supply it. All compatibility issues resolved before any porting or provisioning takes place.
SIP Trunk Provisioning
SIP trunks provisioned and tested in parallel with your live ISDN lines. Call quality verified, failover routes configured, and all settings confirmed before cutover is scheduled.
Number Porting
Porting requests submitted to your current carrier. Port dates agreed and confirmed. All numbers tested on the SIP platform before the ISDN lines are cancelled.
Cutover and Go-Live
Cutover completed outside business hours. IP Four engineers available throughout. ISDN lines remain active until all numbers are confirmed working on SIP. Zero downtime guaranteed.
Post-Migration Support
Full support for 30 days post-migration. Any call quality issues, routing changes, or configuration adjustments handled immediately. ISDN lines cancelled once everything is confirmed stable.
ISDN migrations completed across the UK.
Challenge: A Preston solicitors firm with 8 ISDN2 lines received notice from BT about the 2027 switch-off. They needed to migrate without disrupting client calls or changing their existing Avaya phone system.
Outcome: SIP trunks provisioned and all 8 numbers ported with zero downtime. Existing Avaya PBX retained via SIP gateway. Monthly line costs reduced by 55%. Migration completed in 3 days.
Challenge: A Leeds accountancy firm had ISDN30 with 15 channels and was concerned about the complexity of migrating their DDI range and maintaining call quality during tax season.
Outcome: Full ISDN30 replacement completed outside peak season. All 15 channels and full DDI range ported. Call quality improved with G.711 codec configuration. Annual saving of 8,400 pounds.
Challenge: A Birmingham manufacturer had ISDN lines at their head office and two satellite sites. Each site had different carriers and different switch-off timelines creating coordination complexity.
Outcome: Single SIP trunk provider deployed across all three sites. Coordinated migration completed over two weekends. Free inter-site calls enabled. Total monthly telecoms saving of 1,200 pounds.
Ready to replace your ISDN lines before the 2027 deadline?
We will audit your current ISDN setup and show you exactly what a migration to SIP trunking looks like for your business. Zero downtime, no new phone system required, and costs typically cut by more than half. Free assessment, no obligation.