Introduction
UK businesses now need to plan their switch from ISDN phone lines. Many companies still using these older systems want to make the change smoothly without any call disruptions.
This guide explains ISDN to SIP migration clearly, step by step. It shows exactly what businesses need to do when moving from traditional ISDN lines to modern SIP or VoIP phone systems.
Written for business owners, IT managers and operations teams – this gives you a clear migration process. No sales talk, no technical overload. By the end, you’ll understand how the switch works, what to prepare, and how to complete it confidently.
Why UK Businesses Are Moving Away from ISDN
BT plans to switch off ISDN services across the UK by early 2027 as part of the bigger PSTN changes. Companies still using ISDN for calls, faxes, alarms or payment systems now need to find modern IP-based replacements.
ISDN keeps getting more expensive too. Line rental charges climb higher each year, fixing faults takes longer, and adding or removing lines creates headaches. These issues push most firms towards SIP trunking or VoIP systems that give better control and greater scalability.
For businesses, the ISDN to SIP switch isn’t about chasing new tech. It’s about keeping calls working smoothly and avoiding emergency fixes at the last minute.
Prepare before ISDN switch-off
What Is SIP Trunking and How It Works
SIP trunking allows businesses to make and receive calls using their internet connection instead of traditional phone lines. Rather than physical ISDN circuits, your calls travel across your broadband as data packets.
The SIP trunk connects your existing phone system or cloud platform directly to the normal phone network. You still dial regular landline and mobile numbers exactly as before. The difference lies in the flexibility and scalability at a technical level.
Many people refer to this as ISDN to VoIP migration. SIP trunking specifically keeps your current call handling, phone numbers and routing patterns while adding the scalability most businesses need today.
ISDN vs SIP: Key Differences for Business Phone Systems
When comparing ISDN and SIP, forget the technical terms. What matters are the practical differences for your business.
ISDN needs fixed physical lines and channels. Want more capacity during busy periods? You’ll face engineer visits and extra costs. SIP lets you scale channels up or down instantly to match your actual call volumes.
ISDN ties calls to one location. SIP routes them anywhere – supporting home workers, multiple offices or mobiles without workarounds. SIP also handles call diversion during outages much better than ISDN ever could.
These everyday advantages usually make the decision clear for businesses planning their ISDN switch.
Is Your Current Phone System Ready for SIP?
Before you start planning the migration, first check if your existing phone system can handle SIP trunking.
Most newer PBX systems work with SIP right away they just need some configuration tweaks. Older systems might require a SIP gateway device, or sometimes a complete replacement.
Don’t forget to check connected services too. Door entry systems, lift phones, payment terminals and alarm lines all need reviewing.
Missing this step causes most migration problems later delays and surprise costs that could have been avoided.
ISDN to SIP Migration Steps (Step-by-Step)
The following steps form the core migration process. Each step builds on the previous one and should not be rushed.
Step 1: Audit Your Existing ISDN Lines and Call Usage
Start by making a complete list of every ISDN line, phone number and service your business currently uses. This means main numbers, direct dials, and any non-voice services like alarms or payment systems connected to those lines.
Then review your call patterns – peak volumes, busiest times, and current routing setup. This audit makes sure nothing gets missed during migration and helps you choose the right SIP capacity.
Getting this right from the start creates a solid foundation for the whole ISDN switch.
Step 2: Check Internet Bandwidth and Call Quality Readiness
SIP runs over your internet connection, so bandwidth and stability matter most. Test your current broadband speed, latency and reliability especially during your busiest business hours.
Voice calls don’t need super-fast speeds, but they do need consistent performance. Each call uses roughly 100kbps. If your connection struggles under normal load, consider upgrading or adding a dedicated data line.
This check stops poor call quality problems after you move from ISDN to VoIP.
Step 3: Confirm Phone System or PBX Compatibility
Next, check if your current phone system supports SIP from the start. If it does, look at the manufacturer’s guidelines and any licence requirements.
If your system can’t handle SIP, decide between adding a gateway device or replacing it completely. Think about the system’s age, your growth plans, and ongoing maintenance costs when making this choice.
This step makes sure your technical setup matches your overall business plans for the ISDN to SIP switch.
Step 4: Choose a SIP Trunk Provider
Picking the right provider goes beyond just the cheapest price. Compare them on reliability, their experience with UK number transfers, backup options during outages, and contract terms.
Look for clear pricing with no hidden charges, solid service guarantees, and support teams who understand how critical business phone calls really are.
Choose poorly here and it can ruin even the best-laid migration plans.
Step 5: Plan Number Porting and Migration Timing
Number porting allows you keep all your existing business phone numbers when switching from ISDN. Plan this carefully though porting dates get locked in and delays happen if details aren’t perfect.
Pick a quiet time for the actual switchover. Have backup plans ready just in case. Let your team and customers know what’s happening ahead of time to avoid confusion.
Good timing here cuts the risk of any call downtime during your ISDN to SIP migration.
Step 6: Install, Configure, and Test SIP Trunking
Once your SIP trunks arrive, set up call routing, backup rules and security properly. Testing matters more than anything at this stage.
Check incoming and outgoing calls, voicemail, emergency number routing, and any connections to other systems. Don’t go live until call quality and reliability meets your operational standards.
Step 7: Go Live and Switch Off ISDN
After successful testing and number porting, you can move fully onto SIP and deactivate ISDN services. Monitor call performance closely during the first few days.
Once stability is confirmed, cancel remaining ISDN lines to avoid unnecessary charges. At this point, the ISDN migration is complete.
Plan your migration properly
Common ISDN to SIP Migration Problems (and How to Avoid Them)
The most common issues include underestimated bandwidth needs, incomplete line audits, and rushed number porting. These problems are avoidable with proper planning and realistic timelines.
Security is another consideration. SIP systems should be protected with strong authentication and network controls to prevent misuse.
How Long an ISDN to SIP Migration Takes
The time required for an ISDN to SIP migration depends on system complexity, number of lines, and number porting requirements. While every business is different, most migrations fall within predictable ranges.
- Simple setups (few lines, SIP-ready systems): around 1–2 weeks
- Typical small to mid-sized businesses: usually 2–4 weeks, including number porting
- Larger or multi-site environments: may take longer, especially with phased migrations
Delays are most often caused by incomplete audits or late number porting requests. Businesses that prepare early and follow a structured plan generally complete the migration faster and with fewer issues.
ISDN to SIP Migration Costs Explained
You’ll typically pay for SIP trunk rental, call charges, possible broadband upgrades, and any new hardware your system needs.
Yes, there might be some initial costs, but most businesses save money long-term. Line rental drops and you get much more flexibility.
Looking at the total cost over time tells you more than just comparing headline monthly prices.
Is SIP the Right Choice for Your Business?
SIP trunking works best for businesses wanting to update their phone system while keeping proper call control. If your team prefers fully app-based calling across mobiles and laptops, hosted VoIP might suit better.
Think about how your business actually handles calls day-to-day. That decides whether SIP or a different solution fits your needs best.
How to Choose the Right SIP Provider for ISDN Migration
Don’t pick a SIP provider just because they’re cheap. Look for solid migration experience, reliable service, and proper support instead. They need proven success with UK ISDN number transfers and handling switches in stages.
Make sure they offer flexible call capacity, backup options like call diversion during problems, and clear support processes both during the changeover and afterwards.
Find reliable SIP providers
FAQ
1. Will my existing business phone numbers change after ISDN to SIP migration?
No, in most cases. Your current numbers can transfer over to SIP smoothly. It just needs proper planning and a provider who handles UK number porting well. Checking numbers early avoids any delays.
2. Can my current PBX work with SIP trunking?
Most modern PBX systems handle SIP without problems – just needs setup tweaks. Older ones might need a gateway device or full replacement. Always check compatibility first to avoid surprise costs.
3. What happens to calls if my internet connection goes down?
Good SIP providers offer call diversion to mobiles or backup numbers during outages. This keeps your business reachable even if broadband fails.
4. Is SIP trunking suitable for remote or hybrid teams?
Absolutely. SIP routes calls to any location or device, perfect for home workers and multiple offices. No more being tied to one building.
5. Is SIP more cost-effective than ISDN for UK businesses?
Yes for most companies. SIP cuts line rental costs and lets you adjust capacity easily. Businesses see good savings, especially when call volumes change regularly.
Conclusion
ISDN to SIP migration is now a necessary step for UK businesses, but with proper planning it can be completed smoothly and without disruption. By understanding the migration steps, checking system and connectivity readiness, and planning number porting carefully, businesses can reduce risk and avoid last-minute issues. Once the process is clear, the focus should move to selecting the right system and provider to ensure reliable, flexible communications that support long-term business needs.
