SIP Trunking Explained: What It Is, How It Works & UK Costs

What Is SIP Trunking? A Plain English Explanation

If you’ve been researching business phone systems, you’ve almost certainly come across the term SIP trunking. It sounds technical, but the concept is surprisingly straightforward once you strip away the jargon.

SIP trunking is a method of making and receiving phone calls over the internet instead of through traditional copper telephone lines. SIP stands for Session Initiation Protocol — it’s simply the technology standard that sets up, manages, and terminates voice calls (and video sessions) over an internet connection.

Think of it this way: a traditional phone line is a physical cable running from your office to the telephone exchange. A SIP trunk replaces that physical cable with a virtual connection delivered over your broadband or dedicated internet circuit. The result? The same reliable business calls, but with significantly lower costs, greater flexibility, and far more scalability.

Each SIP trunk provides one or more channels — and each channel supports one simultaneous call. So if your business needs to handle 10 calls at the same time, you need 10 SIP channels. Unlike ISDN, you can add or remove channels almost instantly, without waiting for an engineer visit.

How Does SIP Trunking Work?

Understanding how SIP trunking works is easier when you visualise the key components involved. Here’s a step-by-step overview of the process:

The Key Components

1. Your IP-enabled PBX (phone system): This is the on-premise or virtual phone system that manages your internal calls, extensions, voicemail, and call routing. It must be SIP-compatible (most modern PBX systems are).
2. Your internet connection: A reliable, business-grade broadband or leased line that carries the voice data. Quality of Service (QoS) settings prioritise voice traffic to ensure crystal-clear calls.
3. The SIP trunk provider: A telecoms provider like Connection Technologies delivers the SIP trunk service, connecting your PBX to the public telephone network (PSTN).
4. The PSTN / public telephone network: The wider UK and international phone network that your calls ultimately reach.

The Call Flow — Step by Step

When someone in your office picks up the phone and dials an external number, here’s what happens:

1. Your IP-PBX converts the voice into digital data packets.
2. These packets travel over your internet connection to your SIP trunk provider’s network.
3. The SIP trunk provider routes the call onto the public telephone network (PSTN), connecting you to the recipient — whether they’re on a landline, mobile, or another VoIP system.
4. For incoming calls, the process works in reverse: calls arrive at the SIP provider, are routed down the SIP trunk to your PBX, and ring on the appropriate extension.

The entire process happens in milliseconds. To the caller and recipient, the experience is identical to a traditional phone call — often better, in fact, thanks to HD voice codecs that many SIP providers support.

SIP Trunking vs ISDN vs Hosted VoIP: What’s the Difference?

Choosing the right telephony solution can be confusing. Here’s a clear comparison of the three main options available to UK businesses:

The critical point for UK businesses: BT Openreach is switching off the ISDN and PSTN networks entirely by January 2027. If you’re still using ISDN lines, now is the time to plan your migration. SIP trunking is the natural replacement if you want to keep your existing PBX investment, whilst hosted VoIP is ideal if you’d prefer to move to a fully cloud-based system.

UK SIP Trunking Costs: What Should You Expect to Pay?

One of the biggest advantages of SIP trunking is the cost saving compared to traditional ISDN lines. Here’s a realistic breakdown of UK pricing in 2024/25:

Typical Monthly Costs

• SIP channel rental: £3–£8 per channel per month (the exact price depends on the provider, contract length, and volume)
• Call charges: Often included in bundles, or from as little as 1p/min to UK landlines and 3p/min to UK mobiles. Many providers offer inclusive call packages.
• Number porting: Usually free or a one-off nominal charge to transfer your existing numbers
• Setup fees: Many providers, including Connection Technologies, offer free setup on SIP trunk services

Cost Comparison: SIP Trunking vs ISDN

For a business with 30 simultaneous call channels, the savings are significant:

• ISDN30: Approximately £450–£750 per month for line rental alone
• SIP trunking (30 channels): Approximately £90–£240 per month
• Estimated annual saving: £2,500–£6,000+

That’s before factoring in reduced call charges, the elimination of engineer visit costs, and the flexibility to scale channels up or down as your business needs change throughout the year.

Key Benefits of SIP Trunking for UK Businesses

Beyond the cost savings, SIP trunking delivers a range of strategic benefits:

1. Significant Cost Reduction

As outlined above, businesses typically save 40–60% on their telephony costs when switching from ISDN to SIP trunking. For multi-site organisations, centralising SIP trunks can deliver even greater savings.

2. Instant Scalability

Need extra capacity for a seasonal peak? Add channels in minutes. Downsizing after a project? Remove them just as quickly. There’s no waiting for Openreach engineer appointments or paying for capacity you don’t need.

3. Business Continuity & Disaster Recovery

Because SIP trunks are virtual, calls can be rerouted almost instantly in the event of an office outage, power failure, or broadband issue. Calls can be diverted to mobile phones, an alternative site, or a cloud-based backup system — keeping your business reachable at all times.

4. Keep Your Existing Phone Numbers

Number porting means you can transfer all your existing geographic, non-geographic, and even toll-free numbers to your new SIP trunk service. Your customers won’t notice any change.

5. Future-Proof Your Communications

With the ISDN switch-off approaching in 2027, migrating to SIP trunking now ensures a smooth, planned transition rather than a last-minute scramble.

6. Multi-Site & Remote Worker Support

SIP trunking makes it straightforward to connect multiple office locations under a single telephony platform. Combined with softphones or hosted VoIP solutions, remote and hybrid workers can use the same business phone system from anywhere.

SIP Trunking Setup Requirements: What You Need

Migrating to SIP trunking is far simpler than most businesses expect. Here’s what you’ll need to have in place:

1. A SIP-Compatible PBX

Your existing phone system must support SIP connectivity. Most modern PBX systems from manufacturers like Avaya, Mitel, Panasonic, Samsung, and many others are SIP-compatible. If your PBX is older, an analogue-to-SIP gateway can often bridge the gap at a modest cost.

2. A Reliable Internet Connection

Voice quality depends on your internet connection. As a rule of thumb, each simultaneous call requires approximately 100 Kbps of bandwidth. For a business running 10 concurrent calls, that’s just 1 Mbps — well within the capacity of most business broadband connections. For larger deployments or mission-critical telephony, a dedicated leased line or Ethernet connection is recommended.

3. Quality of Service (QoS) Configuration

Your network router should be configured with QoS settings that prioritise voice traffic over general data. This prevents call quality from degrading during periods of heavy internet usage. Your provider or IT team can configure this quickly.

4. A Reputable SIP Trunk Provider

Choose a UK-based provider that offers resilient infrastructure, UK-based support, transparent pricing, and experience with your specific PBX platform. Connection Technologies ticks all of these boxes, with dedicated account managers and technical support for every SIP deployment.

5. Firewall & Security Configuration

Your firewall needs to be configured to allow SIP traffic (typically on port 5060) and RTP media streams. A good provider will supply detailed configuration guides or assist directly with the setup. SIP-aware firewalls (Session Border Controllers) add an extra layer of security for larger deployments.

Next Steps: Your Complete Guide to SIP Trunking

This article has covered the essentials of what SIP trunking is, how it works, and why it’s the smart choice for UK businesses. For a more in-depth guide — including advanced configuration tips, codec selection, and detailed migration planning — read our comprehensive SIP Trunking Guide for UK Businesses.

If you’re currently on ISDN and unsure where to start, or if you simply want to explore how much you could save, our team is here to help.