What Is SIP Trunking? A Complete Business Guide
What Is SIP Trunking?
SIP trunking is a method of delivering telephone services over the internet using the Session Initiation Protocol (SIP). Instead of relying on traditional ISDN or PSTN copper lines to connect your phone system to the telephone network, a SIP trunk uses your existing broadband connection to carry voice calls as data packets.
Think of a SIP trunk as a virtual phone line. Where a traditional ISDN line is a physical circuit installed by BT or another carrier, a SIP trunk is a digital connection between your PBX (Private Branch Exchange) and a VoIP provider's network. This connection allows you to make and receive calls to any phone number — landlines, mobiles and international numbers — all over your internet connection.
How SIP Trunks Replace ISDN and PSTN Lines
For decades, businesses relied on ISDN (Integrated Services Digital Network) lines to connect their phone systems to the public telephone network. Each ISDN2 circuit provided two simultaneous call channels, while ISDN30 provided up to 30 channels for larger organisations.
With the PSTN and ISDN switch-off completing across the UK, all businesses must migrate to IP-based alternatives. SIP trunking is the most direct replacement — it connects your existing PBX to the telephone network over broadband, preserving your investment in on-premise phone system hardware.
- ISDN2 — replaced by SIP trunks with 2+ channels
- ISDN30 — replaced by SIP trunks with flexible channel counts
- Analogue PSTN lines — replaced by SIP trunks or hosted VoIP
How SIP Trunks Work
When you make a call using a SIP trunk, the following process occurs:
- Your PBX sends a SIP INVITE message to your SIP trunk provider over the internet
- The provider routes the call to the destination number via the public telephone network
- Once the call is connected, RTP (Real-time Transport Protocol) carries the actual audio between both parties
- When the call ends, a SIP BYE message terminates the session
All of this happens in milliseconds, and the call quality is typically indistinguishable from a traditional phone line — provided your internet connection is properly configured.
What You Need for SIP Trunking
To use SIP trunks, you need:
- A SIP-compatible PBX (most modern phone systems support SIP natively)
- A reliable broadband connection with sufficient bandwidth (approximately 100kbps per concurrent call)
- A SIP trunk provider who supplies the trunk, phone numbers and call routing
- Properly configured firewall and network settings (see our firewall configuration guide)
Benefits of SIP Trunking for Business
SIP trunking delivers significant advantages over traditional telephony:
Cost Savings of 30–60%
Businesses typically save 30% to 60% on their telephony costs when switching from ISDN to SIP trunks. You eliminate line rental charges, reduce call costs (especially for international calls), and only pay for the channels you actually need.
Flexible Capacity
Unlike ISDN where you must order circuits in fixed increments, SIP trunks allow you to add or remove channels on demand. Need extra capacity for a seasonal peak? Add channels instantly. Quiet period? Scale back down.
Number Portability
You can port your existing phone numbers to a SIP trunk provider, keeping the numbers your customers know. You can also add numbers from any UK area code, regardless of your physical location.
Geographic Flexibility
SIP trunks work wherever you have an internet connection. This means you can have a single phone system serving multiple offices, or even route calls to remote workers seamlessly.
Business Continuity
If your office loses connectivity, calls can be automatically rerouted to mobile phones, another site, or a voicemail system. This level of resilience is difficult and expensive to achieve with ISDN.
SIP Trunking vs Hosted VoIP
Both SIP trunking and hosted VoIP use the internet for calls, but they serve different needs:
- SIP trunking — best if you already have an on-premise PBX and want to keep it. The SIP trunk simply replaces your ISDN lines.
- Hosted VoIP — best if you want a fully cloud-based phone system with no on-site hardware to manage. The provider hosts everything.
Many businesses are choosing hosted VoIP for its simplicity, while organisations with significant PBX investments or complex call routing requirements often prefer SIP trunking.
Typical SIP Trunk Pricing
SIP trunk pricing in the UK typically includes a per-channel monthly fee (ranging from £3 to £8 per channel) plus call charges. Many providers offer inclusive call bundles. There are no physical installation costs, and setup is usually completed within 24 to 48 hours.
To find out how much your business could save by switching to SIP trunks, get a free quote from our team.
Getting Started with SIP Trunking
If you're considering SIP trunking for your business, start by assessing your current call volumes and concurrent call requirements. Our team can help you determine the right number of channels, port your existing numbers, and configure your PBX for SIP. Request a quote to get started.
