What Is a SIP Trunk? Explained for UK Business Owners
A SIP trunk replaces traditional ISDN phone lines, connecting your existing on-premise PBX system to the public telephone network over your internet connection. This guide explains how SIP trunking works, when it makes sense, and how it compares to full hosted VoIP.
SIP Trunks: The ISDN Replacement
SIP trunks replace ISDN lines by carrying business phone calls over an IP connection — keeping your existing PBX whilst eliminating expensive ISDN line rental. With BT's PSTN switch-off deadline of 31 January 2027, businesses with ISDN connections must migrate. SIP trunking is typically 30–50% cheaper than ISDN and can be deployed without replacing the existing phone system.
Explore business VoIP optionsWhat Is a SIP Trunk?
A SIP (Session Initiation Protocol) trunk is a virtual telephone line that connects a business's on-premise PBX (Private Branch Exchange) phone system to the public telephone network over an internet or dedicated data connection. Where a traditional ISDN line carries voice calls over a physical copper circuit, a SIP trunk carries those calls as data packets over an IP connection.
In practice, SIP trunking allows a business to keep its existing PBX hardware and phone extensions whilst replacing the expensive ISDN lines that connect the system to the outside world. This makes SIP trunking the most cost-effective migration route for businesses with a relatively modern, compatible PBX that they do not want to replace as part of the PSTN switch-off.
How SIP Trunking Works
A SIP trunk provider allocates a set of virtual channels — each channel supports one simultaneous call. When a staff member dials out, the call is established over the data connection to the SIP provider, which then connects it to the public telephone network. Incoming calls to your phone numbers arrive at the SIP provider and are routed to your PBX over the same data connection.
The PBX handles internal call routing, hunt groups, auto-attendants, and voicemail as it always has — the SIP trunk simply replaces the ISDN connection to the external network. Most modern PBX systems — from Avaya, Cisco, 3CX, Mitel, and others — support SIP trunking natively or with a straightforward software upgrade.
SIP Trunking vs Hosted VoIP
SIP trunking retains your on-premise PBX hardware. The trunk provides the external connection, but your existing system still manages everything internally. This is the right choice if your PBX is relatively modern, has features your business relies on, and has useful remaining life — typically five or more years remaining before it would need replacing anyway.
Hosted VoIP (or UCaaS) moves the entire phone system to the cloud — there is no on-premise PBX at all. Calls are managed by the provider's platform, accessed via desk phones, softphones, or mobile apps. Hosted VoIP has lower upfront cost, is more flexible for remote working, and removes the need to manage PBX hardware. For businesses with an ageing PBX that needs replacement regardless, hosted VoIP is often the more cost-effective long-term solution.
Why UK Businesses Are Moving to SIP Trunking
The BT PSTN switch-off — scheduled for 31 January 2027 — is the primary driver. All ISDN connections will stop working on that date. SIP trunking is one of two main migration routes: either replace ISDN with SIP trunks (retaining the PBX) or replace the entire phone system with hosted VoIP. Europe's SIP trunking market is growing at approximately 9.8% CAGR as businesses across the continent make this transition.
Beyond the deadline, SIP trunking delivers ongoing cost benefits. ISDN line rental is typically £25 to £50 per channel per month; SIP channels from a business provider cost significantly less, with lower per-minute call rates as well. Businesses with 10 or more ISDN channels often see monthly savings that justify the migration cost within 12 months.
Key Considerations for UK SMEs
- Check PBX compatibility first: Not all PBX systems support SIP natively. Some require a gateway device or firmware upgrade to work with SIP trunks. AMVIA assesses PBX compatibility as part of the migration planning process.
- Ensure sufficient broadband bandwidth: Each simultaneous SIP call requires approximately 80Kbps of upload and download bandwidth. For a business with 10 concurrent calls, that is roughly 1Mbps upload — manageable on most FTTC or FTTP connections, but should be checked.
- Configure quality of service (QoS): Router-level QoS settings prioritise voice traffic over bulk data, preventing call quality issues when the connection is under load.
- Plan number porting: Geographic numbers can be ported to the new SIP provider. Allow 7 to 14 working days for the port — plan this in advance to avoid any gap in service.
How AMVIA Can Help
AMVIA provides managed SIP trunking services for UK businesses, including PBX compatibility assessment, number porting, configuration, and ongoing management. Where a business is also considering a full hosted VoIP migration, AMVIA will assess both options and make an honest recommendation based on your existing system, team size, and long-term requirements. Call 0333 733 8050 to discuss your PSTN migration options.
SIP Trunking: Key Features
What SIP trunks provide compared to traditional ISDN connections.
ISDN Replacement
Drop-in replacement for ISDN2 and ISDN30 lines — keep your existing PBX and phone numbers.
Flexible Channel Scaling
Add or remove simultaneous call channels without engineer visits — scale with your business.
Cost Reduction
Typically 30–50% lower cost than ISDN line rental with lower per-minute call rates.
Number Portability
Retain existing geographic phone numbers — no disruption to clients who already know your number.
SIP Trunk Implementation Checklist
What to confirm before migrating from ISDN to SIP trunking.
PBX compatibility confirmed
Existing PBX supports SIP natively or via a compatible gateway device.
Broadband bandwidth assessed
Upload speed sufficient for maximum expected concurrent calls — typically 1–2Mbps for 10–20 channels.
QoS configured on router
Voice traffic prioritised to prevent call quality issues under load.
Number porting planned
All geographic numbers identified and porting arranged — 7–14 working days lead time.
Channel count determined
Number of simultaneous call channels based on peak usage analysis — not total extensions.
Disaster recovery considered
Failover plan in place if internet connection fails — calls can divert to mobiles automatically.
SIP Trunk FAQs
It depends on the age and condition of your PBX. If your PBX is relatively modern (less than five years old), has features your business relies on, and is SIP-compatible, trunking is often the most cost-effective migration path. If your PBX is approaching end of life, needs expensive maintenance, or lacks features like mobile apps and Teams integration, hosted VoIP is likely the better long-term investment. AMVIA will assess both options honestly based on your specific situation.
The number of SIP channels you need equals the maximum number of simultaneous calls your business makes at any one time — not the total number of extensions. Most businesses require significantly fewer channels than extensions. For example, a 30-person business typically needs 8 to 12 SIP channels. AMVIA analyses your current call data to recommend the right number of channels, avoiding over-provisioning costs.
Yes. Number porting transfers your existing geographic numbers to your new SIP trunk provider. The process takes 7 to 14 working days. During the porting window, calls continue to route through your existing ISDN lines until the port completes — so there is no gap in service. AMVIA manages number porting as part of its SIP trunk installation service.
Each simultaneous call requires around 80–100Kbps of upload and download bandwidth. For most UK businesses on FTTC or FTTP broadband, bandwidth is not the limiting factor. More important is connection stability — jitter and packet loss affect call quality more than raw speed. AMVIA configures QoS on your router to prioritise voice traffic, and recommends whether your current connection is suitable for SIP trunking before installation.
Migrate from ISDN to SIP Trunks
AMVIA will assess your existing PBX, check compatibility, and manage the full SIP trunk migration — including number porting and QoS configuration.
Related Resources
PSTN Switch-Off: What You Need to Do
Everything UK businesses need to know about the January 2027 PSTN switch-off deadline.
What Is Business VoIP?
How VoIP works and how it compares to traditional phone lines for UK businesses.
UCaaS for UK SMEs
When a fully hosted cloud phone system is the better alternative to SIP trunking.
Business VoIP Solutions
AMVIA's full range of managed VoIP services for UK businesses.