Verified against Ofcom Connected Nations Spring 2026, IASME Cyber Essentials April 2026 standard and current operator pricing.
Quick answer: SIP (Session Initiation Protocol) is the universal standard that makes VoIP calls happen. It’s the part of the system that says “ring this phone, hang up that phone, transfer this call”. Almost every UK business phone system in 2026 — from Microsoft Teams Phone to RingCentral to a £6/month hosted VoIP plan — uses SIP under the hood. You don’t need to operate it directly to use it, but understanding it helps you ask the right questions when buying.
SIP in one sentence
SIP is a text-based signalling protocol — a bit like HTTP for phone calls — that tells your VoIP equipment when to ring, when to pick up, who’s calling whom, and when to hang up. The actual audio of the call rides on a separate protocol called RTP, but SIP is the air-traffic controller that makes the whole thing work.
SIP PhoneHosted PBXPSTN / SIP Trunk
SIP signalling flows between your phone, the hosted PBX and the SIP trunk that connects to the public network.
The full name (and what each word actually means)
- Session — a phone call (or video call, or screen share). Anything with a start and an end.
- Initiation — SIP’s job is to start, modify and end sessions. It doesn’t carry the voice itself.
- Protocol — a published standard. Defined in RFC 3261 by the IETF in 2002, refined a hundred times since.
What SIP actually does on a call
Walk through a typical “Bob calls Alice” interaction:
- 1 · INVITE
Bob’s phone sends an INVITE
Bob’s IP phone tells the hosted PBX: “I want to call alice@company.com”. The PBX looks her up.
- 2 · 100 Trying
PBX acknowledges
The PBX responds with “100 Trying” so Bob’s phone knows it’s not been ignored.
- 3 · 180 Ringing
Alice’s phone rings
PBX forwards the INVITE to Alice’s handset. Her phone responds with “180 Ringing”, which travels back to Bob — that’s why he hears a ringtone.
- 4 · 200 OK
Alice picks up
Her phone sends “200 OK” with codec preferences. Bob’s phone responds with ACK. Now SIP has done its job and steps aside.
- 5 · RTP audio
Audio flows over RTP
Voice packets stream directly between the two phones (or via the PBX). Each ~20ms of audio is one tiny UDP packet.
- 6 · BYE
Either side hangs up
Whoever ends the call sends “BYE”, the other side replies “200 OK”, session closed.
SIP vs SIP trunking — they’re not the same thing
The two terms are easy to confuse. Quick clarification:
SIP (the protocol)
The language that VoIP equipment uses to set up and end calls. Free, open, defined by the IETF.
You experience SIP through:
- Hosted VoIP apps (RingCentral, 8×8)
- Microsoft Teams Phone
- SIP desk handsets (Polycom, Yealink)
- WebRTC browser calls
SIP trunking (a service)
A virtual phone line you buy from a carrier, replacing ISDN. Connects an on-premise PBX to the public phone network using SIP.
You buy SIP trunks if you:
- Already own a PBX you want to keep
- Need 30+ concurrent channels
- Want carrier-grade DDI ranges
- Are migrating off ISDN30 before the 2027 switch-off
For most UK SMBs, you don’t buy SIP — you buy hosted VoIP, and the provider deals with SIP internally. Only larger businesses with on-premise PBXs buy SIP trunks directly. Read more in our SIP trunking guide for UK businesses.
Ready to switch to VoIP?
SIP-based hosted VoIP from £12/user/month. Free setup, number porting and UK support. Quote in 60 seconds.
Or call us on 0333 015 2615
Why UK businesses care about SIP in 2026
Three reasons SIP is suddenly relevant for non-techies:
The 2027 PSTN switch-off
BT is decommissioning every analogue and ISDN line by January 2027. SIP-based services are the only future-proof replacement.
Hybrid working needs IP
SIP lets the same phone number ring a desk handset, a laptop softphone and a mobile app simultaneously. ISDN couldn’t do that.
Cost
SIP-delivered calls cost typically 50% less than ISDN equivalents. Line rental disappears entirely with hosted VoIP.
SIP requirements — what you actually need
- A reliable internet connection — 100 kbps per simultaneous call (G.722 HD codec).
- QoS-capable router if voice and data share the same line.
- SIP-aware firewall — or critically, one with SIP ALG disabled (see our SIP ALG guide).
- SIP-compatible handsets or apps — Yealink, Polycom, Cisco, Snom for hardware; the provider’s softphone for laptop/mobile.
- A SIP account — supplied by the hosted VoIP provider or SIP trunk carrier.
SIP security — is it safe?
Plain SIP is sent in clear text and can theoretically be intercepted. UK business providers in 2026 default to encrypted SIP using two complementary standards:
| Layer | Standard | Protects | Default in 2026? |
|---|---|---|---|
| Signalling | SIP-TLS (port 5061) | Who is calling whom, dialled digits | Yes on hypercloud, RingCentral, 8×8, Teams Phone |
| Voice payload | SRTP | The actual audio of the call | Yes on the same providers |
Plus standard hygiene: strong SIP credentials, SIP password rotation, IP whitelist for SIP trunks, fraud monitoring (premium-rate alerts), and disabling SIP ALG on the router. Done right, SIP is more secure than the analogue line it replaces.
Switching to SIP-based phones before the 2027 switch-off?
We migrate UK businesses from ISDN to hosted VoIP or SIP trunks every week. 30-minute discovery, no sales pressure.
SIP — FAQs
No — SIP is for connecting to the public phone network with real phone numbers. WhatsApp and FaceTime are over-the-top apps with their own walled-garden numbering. SIP is what business phones use because it can dial any 11-digit UK number.
Yes — most modern PBXs (Avaya, Mitel, 3CX) support both. This is how UK businesses migrate gradually before the 2027 ISDN switch-off.
5060 for unencrypted SIP, 5061 for SIP-TLS, and a range like 10000–20000 UDP for the RTP audio. If your firewall blocks those ports outbound, calls fail.
SIP ALG is supposed to help SIP traverse NAT — but in practice it usually breaks calls (one-way audio, dropped registrations). Most UK VoIP providers recommend disabling it. Our SIP ALG explainer covers exactly how.
Related Reading
More from the Connection Technologies blog.