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 by 2027, 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)

SIP Trunking Costs

Understanding what SIP trunking costs is essential for budgeting your migration. Here is a breakdown of typical UK pricing in 2026:

Cost Element	Typical Range	Notes
Per-channel fee	£3–8/month	Each channel = one concurrent call
UK landline calls	0.5–1.5p/min or inclusive	Many providers bundle unlimited UK calls
UK mobile calls	2–5p/min or inclusive	Bundled in most business packages
Number porting	Free–£5 per number	Most providers port numbers for free
Setup fee	£0–50	Often waived; no engineer visit needed
International calls	1–10p/min	Far cheaper than ISDN international rates

Compared to ISDN, where line rental alone can cost £25–35 per channel per month before call charges, SIP trunking delivers substantial savings — especially for businesses making high volumes of calls. For a detailed cost comparison, see our SIP trunking UK guide for 2026.

SIP Trunking vs ISDN

With the ISDN switch-off approaching, understanding the differences between SIP trunking and ISDN helps you plan your migration:

Feature	SIP Trunking	ISDN
Delivery	Over broadband (IP)	Dedicated copper circuit
Cost per channel	£3–8/month	£25–35/month
Scalability	Add/remove channels instantly	Fixed increments, engineer visit
Setup time	24–48 hours	2–6 weeks
Call quality	HD voice (wideband)	Narrowband only
Failover	Automatic rerouting to mobiles/backup	Limited; requires additional circuits
Future availability	Long-term supported	Switched off by 2027

The bottom line: ISDN is a legacy technology being retired. SIP trunking offers lower costs, better flexibility and HD call quality — and it is the technology that will be supported going forward. Read more about the PSTN switch-off and what it means for your business.

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.

Getting Started with SIP Trunking

If you are considering SIP trunking for your business, here is a step-by-step approach:

1. Audit your current setup — how many concurrent calls do you need? What PBX do you have?
2. Check your broadband — ensure you have sufficient bandwidth (100kbps per concurrent call) and consider a dedicated connection for voice traffic
3. Choose a provider — look for UK-based support, number porting, and flexible contracts
4. Configure your firewall — follow our SIP trunk firewall configuration guide to ensure traffic flows correctly
5. Port your numbers — your provider will handle the porting process, typically taking 5–10 working days
6. Test and go live — run parallel testing before cutting over fully