How to Migrate from ISDN to VoIP: A Step-by-Step Guide for UK Business
Migrating from ISDN to VoIP means moving your business calls off the old ISDN lines onto an internet-based phone system before the network closes on 31 January 2027. It happens in five stages: audit your current setup, check your internet is ready, choose a VoIP solution, port your numbers, then test and go live.
Done properly, the switch completes with no loss of service. This guide walks through each step, how long it takes and what it costs.
How to migrate from ISDN to VoIP in 5 steps
To migrate from ISDN to VoIP, audit your existing lines and connected devices, check your internet can carry the calls, choose a VoIP solution, port your phone numbers, then run parallel tests before ceasing the old ISDN service. Each step is below.
Audit your current ISDN setup
List every active ISDN2 and ISDN30 line, all your direct dial-in (DDI) numbers, and any fax lines. Map your call routing, hunt groups, auto-attendants and voicemail. Then check every device tied to a phone line: fax machines, fire and intruder alarms, lift emergency phones, door entry and card terminals. This audit is your migration plan, and the non-voice devices are what catch businesses out.
Check your internet and network are ready
VoIP calls run over your internet connection, so it has to carry them. Allow roughly 100 kbps per concurrent call, so a 20-call office needs about 2 Mbps of headroom on top of normal use. Confirm your broadband is digital-ready (full fibre or SoGEA), and set Quality of Service (QoS) on the router so voice takes priority over other traffic. For business-critical sites, add a backup connection.
Choose your VoIP solution and provider
Decide your migration path. Hosted VoIP replaces the whole phone system with a cloud service and needs no hardware on site, which suits most businesses. SIP trunking keeps a modern on-site PBX and swaps only the lines. Then choose your provider on UK-based support, uptime guarantees and the features you need: mobile apps, call recording and CRM integration.
Port your phone numbers
Your existing numbers move with you. Sign a Letter of Authorisation (LoA) and supply a recent provider invoice with your full number list. Keep the ISDN lines active during the porting window, which typically runs one to three weeks, so nothing drops. Never cease the ISDN line before the port completes, because a number attached to a ceased line can be difficult or impossible to recover.
Test, train and go live
Run the new VoIP system alongside the old lines. Test every ported number, extension, voicemail and emergency routing path, and confirm any fax runs over T.38. Train the team on the apps and handsets. Only once everything works, and after a short parallel period, cease the ISDN contract. This is how the switch happens with no downtime.
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Is your business ready for the UK Big Switch Off? Moving from ISDN to cloud VoIP.
How long does ISDN to VoIP migration take?
A straightforward single-site migration usually completes in two to six weeks. Multi-site businesses with number porting, new connectivity and special services typically take one to three months. The two things that add time are full fibre installation lead times and number porting, so start early rather than close to the 2027 deadline when engineer slots are scarce.
How to migrate from ISDN to VoIP without downtime
The switch happens with no downtime when the old and new systems run in parallel. Keep the ISDN lines live, set up the VoIP system alongside them, test every number and route, then port the numbers and cease ISDN only after the new system is proven. Porting last, not first, is what protects your calls.
What does ISDN to VoIP migration cost?
Most UK businesses pay £8 to £20 per user per month for hosted VoIP once migrated, usually including calls, maintenance and updates that ISDN rental and PBX upkeep did not. At The VoIP Shop, business plans start from £13.95 per user per month with UK call bundles, 24/7 UK-based support and a dedicated account manager. Migration itself, including number porting, is planned and priced up front with no surprise fees.
What are the common pitfalls, and how to avoid them?
Three mistakes cause most migration problems: ceasing the ISDN line before the numbers have ported (avoid by porting first, ceasing last); forgetting the non-voice devices like alarms and lift lines (avoid by auditing every socket in Step 1); and under-provisioned broadband (avoid by checking bandwidth in Step 2). A proper audit and a parallel-running cutover remove all three.
ISDN to VoIP migration FAQs
How do I migrate from ISDN to VoIP?
How long does it take to migrate from ISDN to VoIP?
Can I migrate from ISDN to VoIP without downtime?
Will I keep my phone numbers when I migrate from ISDN to VoIP?
How much does it cost to migrate from ISDN to VoIP?
What internet do I need to migrate from ISDN to VoIP?
What happens to my alarms and card machines when I migrate?
Get your free ISDN to VoIP migration plan
We will audit your ISDN lines, your connected devices and your broadband, then give you a written migration plan with a fixed monthly cost and no obligation. For over 10 years we have moved UK businesses off ISDN with 24/7 UK-based support.
Written By | About the Author
Kully Hothi
Sales Director
Kully Hothi is Sales Director at The VoIP Shop and one of the company's lead authors. He has worked in UK telecoms since 2011, and has taken businesses of all sizes through the move off ISDN and analogue lines onto cloud phone systems, porting numbers and cutting over without downtime. He is sales certified across 3CX, Yeastar and Cisco as well as Openreach and all four major UK mobile networks, which is why he looks at phones, mobiles and connectivity together rather than as three separate problems. More about Kully Hothi.
