| Feature | ISDN | SIP Trunking |
| Delivery method | Physical copper lines | Internet (broadband) |
| Monthly cost | Higher line rental | Significantly lower |
| Call channels | Fixed — one per line | Flexible — scale as needed |
| Installation | Engineer visit required | Remote configuration |
| Adding capacity | Order new lines, wait weeks | Add channels in minutes |
| Switch-off risk | Yes — stops January 2027 | No — internet-based |
| Number porting | Yes | Yes — numbers transfer |
| PBX compatibility | Works with any PBX | Requires IP-capable PBX |
| Disaster recovery | Limited | Call divert and failover built in |
ISDN replacement · Keep your existing PBX · Cost reduction
SIP trunking — replace your ISDN lines without replacing your phone system
If your business has a modern IP-capable PBX that you're not ready to replace, SIP trunking lets you keep it. Instead of replacing your entire phone system, you simply swap your ISDN lines for SIP trunks — internet-based connections that do the same job at lower cost and without the January 2027 deadline hanging over you.
✓Keep your existing PBX
✓Lower line rental costs
✓ISDN switch-off ready
✓Edinburgh setup & support
SIP Trunking in plain English
What is SIP trunking and how does it work?
SIP stands for Session Initiation Protocol, the technical standard that governs how voice calls travel over the internet. A SIP trunk is essentially a virtual phone line: it connects your existing PBX to the telephone network via your broadband connection rather than through a physical ISDN cable.
Where ISDN gives you a fixed number of physical channels, each one capable of carrying one call, SIP trunks are flexible. You can have as many or as few concurrent call channels as your business needs, and scale up or down without any hardware changes.
For businesses that already have a capable IP-PBX, SIP trunking is the most cost-effective path to ISDN switch-off compliance. You're not replacing your phone system, you're replacing the lines that feed it. The phones, the extensions, the call routing your team is familiar with, all of that stays exactly as it is.
How they compare
SIP trunking vs ISDN — what changes and what stays the same
Why businesses choose SIP trunking
What SIP trunking gives your business
Cost savings
Meaningful cost reduction. SIP trunks cost significantly less than ISDN lines — both in monthly line rental and in call charges. Businesses typically see savings of 40–60% on their line costs when switching from ISDN to SIP, with no reduction in call quality or capacity. For businesses paying for multiple ISDN channels they don't fully use, the savings can be immediate and significant.
Protect your PBX investment
Keep your existing system. If your business has invested in a capable IP-PBX in recent years, replacing it ahead of schedule is an unnecessary cost. SIP trunking lets you protect that investment — the PBX stays, the ISDN lines go, and your phone system continues working as it always has, just over the internet instead of copper.
Flexibility
Scale capacity on demand. ISDN channels are fixed — if you need more concurrent calls, you order more lines and wait. SIP trunks are virtual, so adding or removing call channels is a configuration change. For businesses with seasonal peaks, project-based call volumes or rapid growth, this flexibility has real operational value.
Future-proofed
Switch-off compliant. SIP trunking resolves the ISDN switch-off deadline entirely. Once your lines are migrated to SIP, the January 2027 deadline is no longer a concern — your phone system is running on internet-based infrastructure that isn't going anywhere.
Is SIP trunking right for your business?
The businesses that benefit most from SIP trunking
SIP trunking is a more targeted solution than VoIP or hosted PBX — it's the right answer for a specific situation rather than a general-purpose recommendation. It works best for:
- Businesses with a modern IP-PBX already in place — if your PBX was installed or upgraded in the last five to eight years, there's a reasonable chance it's already SIP-compatible. Rather than replacing the whole system ahead of the switch-off, SIP trunking lets you keep what you have and just swap the lines.
- Businesses that have recently invested in their phone system — if you've spent money on your current PBX in the last few years, replacing it now feels wasteful. SIP trunking extends the working life of that investment until you're ready to upgrade on your own terms rather than under deadline pressure.
- Businesses with fluctuating call volumes — the ability to scale SIP channels up and down quickly makes SIP trunking particularly valuable for businesses whose call volume varies — by season, by project or by growth stage. You pay for the capacity you need, not a fixed number of physical lines.
- IT-managed businesses — businesses with an internal IT function or an IT partner already managing their PBX often prefer SIP trunking because it keeps the existing system architecture intact. Your IT team or partner continues managing the PBX; we manage the SIP trunks and the connection to the telephone network.
Before you commit
Is your existing PBX compatible with SIP trunking?
Not every PBX can work with SIP trunks. Older proprietary systems — particularly those designed specifically for ISDN — may not support SIP without significant modification or a hardware upgrade, which can make SIP trunking less cost-effective than simply migrating to a hosted system.
As a rough guide, your PBX is likely SIP-compatible if:
- It was installed or significantly upgraded within the last five to eight years
- It's described as an "IP PBX" or "hybrid PBX" in the documentation
- It already has IP phones or softphones connected alongside any digital handsets
- Your IT team or supplier has previously mentioned SIP compatibility
Your PBX is less likely to be SIP-compatible if it's an older proprietary system (Avaya, Mitel, Panasonic or similar from more than eight years ago) connected exclusively via ISDN channels.
The easiest way to find out is to ask us. We'll assess your current system — usually from the model number and configuration details — and tell you whether SIP trunking is viable, what any modifications would cost, and whether it makes more financial sense than migrating to a hosted PBX instead.
How it works
Moving from ISDN to SIP — what to expect
Compatibility check
We confirm your PBX is SIP-compatible and identify any configuration changes needed. We'll also check your broadband connection has sufficient capacity and quality for the number of simultaneous calls you need to support.
Number porting
We initiate the porting of your existing numbers to the SIP platform. Your numbers stay exactly the same — we just change where they route to. Porting is coordinated to minimise any service interruption, typically happening overnight.
SIP configuration
We configure your PBX to connect to the SIP trunks, set up your call channels and test everything thoroughly before going live. If your PBX requires any firmware updates or configuration changes to support SIP, we handle those as part of the migration.
Go-live & monitoring
On go-live day your ISDN lines are disconnected and SIP trunks take over. We monitor the transition and are available throughout to deal with anything that comes up. Most migrations complete without any noticeable interruption to service.
SIP trunking questions
How many SIP trunks does my business need?
+Will call quality be affected when switching to SIP?
+What happens to our ISDN lines during the migration?
+Can I add more SIP trunks later if my business grows?
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Want to know if SIP trunking is right for your business?
The quickest way to find out is a short conversation. Tell us about your current phone system and we'll tell you whether SIP trunking makes sense, what it would cost, and how it compares to a full hosted PBX migration. No jargon, no obligation.