SIP Trunking explained: what it is, why it matters, and how it works in 2026

SIP trunking is now the standard way UK businesses connect their phone system to the public telephone network. It replaces the old analogue lines and ISDN circuits BT spent decades selling — most of which are being switched off as part of the UK PSTN retirement.

But SIP isn't just a cheaper ISDN. The real win is flexibility: a single trunk can carry any number of UK and international numbers, scale up and down on demand, and follow your team wherever they work. It is upgrading managed VoIP because the working environment is genuinely better — not because the per-minute rate is lower.

What is SIP trunking?

SIP (Session Initiation Protocol) is the signalling standard the telecoms industry uses to set up voice calls over IP networks. A "trunk" is the virtual connection between your phone system — your PBX, softphone app, or IP desk phone — and a licensed carrier's network. Calls travel over your regular broadband or leased line rather than a dedicated copper or fibre voice circuit.

SIP trunking is now natively supported by every modern PBX (3CX, Asterisk, FreePBX, Cisco, Avaya, Mitel and so on) and by a wide range of adapters for older equipment. The carrier — that's us — handles the regulated interconnect to other UK telcos so your calls land on any phone in the country.

Why would a business want SIP trunking?

Three reasons most businesses move to SIP:

Cost. Calls run over IP, not the public telephone network. There's no per-line rental and no separate "voice circuit" to pay for. Wholesale call rates are passed through at carrier cost — with NumberGroup Membership that's roughly 0.3p/min for UK landlines and 0.6p/min for UK mobiles (rounded, includes the connection charge). See the full wholesale call rate card for international and special-rate destinations.

Flexibility. Add, remove or move numbers any time — email or call the team and it's done, usually same day. Run 0800 freephone, 03 UK-wide and any UK geographic area code on the same trunk. Send your existing number out (CLI) on calls from anywhere — handy if you're working from home but want the office number to display.

Resilience. Internet outages don't mean phones down. Routing rules at the carrier level can redirect inbound calls to a mobile, a different office, or a voicemail-to-email box the moment a trunk goes offline. That isn't possible on traditional ISDN.

What sound quality does SIP trunking provide?

As a minimum, SIP delivers ISDN-equivalent sound quality. Where both ends are SIP and the codec supports it (G.722, Opus), calls can be HD voice — noticeably clearer than a traditional landline. Quality depends almost entirely on the broadband link: prioritise voice traffic on your router (QoS) and SIP holds up fine on even modest connections.

Don't confuse business SIP with peer-to-peer apps or pre-paid consumer VoIP, which often use cut-price routing and degraded codecs to keep margins up. Licensed carriers like us route over our own network and the regulated UK wholesale fabric.

How reliable is the service?

Our network is built on fully redundant infrastructure with automatic failover. If a route drops, calls re-route through an alternate path without you noticing. Inbound numbers can be configured to fall back to a mobile or voicemail automatically if your trunk is unreachable — something traditional ISDN simply can't do.

Will my customers notice anything different?

No. The experience for the person ringing you is identical — normal ring tones, normal voice quality, no perceptible delay. You keep using the same handsets if you want. What changes is on your side: smart features become possible. IVR menus, hunt groups, time-of-day routing, call recording, voicemail-to-email and missed-call alerts are all included with Membership rather than charged as bolt-ons.

Are emergency services supported?

Yes. Calls to 999 are supported. We hold an address against each line so emergency operators see the right location. The only caveat is the obvious one: SIP needs power and a working internet connection at your end. If both go down (a fire, a power cut), mobile is your fallback — the same is true of any modern phone system, including the all-IP replacement BT is rolling out as ISDN is switched off.

What equipment do I need?

One of the following:

An IP PBX — physical (Cisco, Mitel) or software (3CX, Asterisk, FreePBX). The PBX registers with our SIP service and routes calls to extensions.

IP desk phones — register them directly with our service, or front them with a PBX. We supply Grandstream models at our distributor cost (no handset markup) for Members.

A softphone — apps that run on a laptop or mobile. We provide branded Acrobits softphones at supplier cost: pay per active install, not per named seat.

We don't sell broadband, but we're happy to talk through what bandwidth you need or recommend a partner who supplies business connectivity.

Can we keep our existing numbers?

Yes. We can port any UK telephone number from any other UK provider onto our SIP service. Port-in is free with Membership. While porting is in progress (typically 5–10 working days, sometimes longer for older or more complex setups), nothing changes on the caller's side — your number works exactly as before. The cut-over happens within a defined window agreed in advance.

You can also send any number you already own out as your CLI without porting it to us — useful for businesses with split providers.

Can we have a different telephone number?

Yes — and with Membership, as many as you like. From the self-care portal you can assign UK geographic numbers in any area code (London 020, Manchester 0161, Edinburgh 0131, every local 01/02), 0800 and 0808 freephone, 03 UK-wide non-geographic, or 084 business numbers. See the numbers included with Membership.

Can I hide my number from the people I'm calling?

Yes. SIP supports presentation control at the protocol level, and you can also override it per-call by dialling 141 before the number to withhold or 1470 to reveal. Note that 999 and 112 always see your number regardless.

Do you support encryption?

Yes. Our SIP service supports both encrypted signalling and encrypted media. We use high-strength TLS for SIP registration and call control, and optional SRTP to encrypt the voice stream itself. Both are widely supported by commercial IP PBXs and by open-source stacks like FreePBX and Asterisk.

Do you provide voicemail?

Yes. Every inbound feature works on SIP trunks — voicemail, voicemail-to-email (MP3 attachment with caller details), call recording, IVR menus, hunt groups, time-of-day routing. All included with Membership. No bolt-on fees.

What does SIP trunking cost?

With NumberGroup Membership at £149/month flat:

SIP channels: Free (up to 25 concurrent calls included).
Standard UK numbers: Free (unlimited).
Port-in of existing numbers: Free.
Smart features (IVR, recording, routing, voicemail): All-in.
UK landline calls: ~0.3p/min (carrier cost).
UK mobile calls: ~0.6p/min (carrier cost).

Need more than 25 concurrent calls? Get in touch — we'll quote based on your real peak. We don't do per-channel licence fees and we don't do minimum-commit contracts.

How do I start using NumberGroup SIP trunking?

If you're already a Member, just ask us to set up a SIP trunk — you don't provision it yourself. We create the trunk, point your numbers at it, and send you the credentials to configure your PBX or handset. Two minutes if you know what you're doing — and our team will walk you through it if you don't.

Not a Member yet? Start with Membership — £149/month, no contract, port your numbers in for free, and our UK team will get your PBX talking to us before your first invoice lands.