What Is SIP Trunking? A Guide to Business SIP Trunks

ISDN is being switched off across the UK, and most businesses replacing it are moving to SIP trunking. Here's what SIP trunking actually is, how it works with an existing phone system, what matters when comparing providers, and where Plustel's own SIP trunks fit in.

What Is SIP Trunking?

SIP stands for Session Initiation Protocol — the technology that sets up, manages and ends a voice call over the internet. A "SIP trunk" is a virtual phone line that uses this protocol to connect your existing phone system (PBX) to the wider telephone network over your internet connection, instead of over traditional copper ISDN lines.

In practice, this means your business keeps the phone system and handsets it already has, but the physical ISDN lines going into the building are replaced with a data connection carrying voice traffic. Each "channel" on a SIP trunk is roughly equivalent to one simultaneous call, and channels can usually be added or removed far more easily than physical ISDN lines ever could.

Why Businesses Are Moving to SIP Trunks

The main driver right now is straightforward: ISDN is being retired. Openreach has been phasing out the ISDN network across the UK, and providers are no longer able to install new ISDN lines. Businesses still on ISDN need to move to an alternative before their existing lines stop being supported — and SIP trunking is the direct, like-for-like replacement, since it works with the PBX hardware most businesses already have rather than requiring a full system replacement.

Beyond simply replacing ISDN, SIP trunking tends to bring genuine improvements:

• Lower cost per channel — SIP channels are typically cheaper than ISDN lines, and calls often cost less too.
• Flexible capacity — channels can usually be added or reduced to match demand, rather than being fixed to the physical lines installed.
• Number portability and geographic flexibility — numbers aren't tied to a physical line at a specific address in the same way.
• Built-in resilience — a good SIP trunk provider offers automatic failover, so an internet outage at one location doesn't necessarily mean lost calls.

What to Look For in a SIP Trunk Provider

SIP trunking is fairly standardised in principle, but providers differ a lot in practice. Worth checking before switching:

• Number porting support — can they port your existing numbers across, and how straightforward is the process?
• Self-service provisioning — can you make changes (adding channels, updating routing) yourself through a portal, or does every change need a support call?
• Automatic failover — what happens to incoming calls if your primary connection goes down? A trunk without failover just stops working.
• Included features — call recording and number barring are worth having as standard rather than a paid extra.
• Compatibility with your PBX — most modern SIP trunks work with most modern PBX systems, but it's worth confirming compatibility with your specific setup before switching, particularly with older on-site hardware.

How Plustel's SIP Trunks Compare

Plustel's business SIP trunks are built to be flexible for exactly the reasons above: number porting and self-provisioning are handled through our own portal, and every trunk includes call recording, number barring and automatic failover as standard rather than as add-ons. Multi-channel options mean capacity can scale with a business rather than being fixed at setup.

Because Plustel runs its own switch rather than reselling someone else's platform, faults and changes get handled directly, without being passed up a longer support chain. Whether you're a business replacing ISDN before it's switched off, or a reseller adding SIP trunking to your own product range, the SIP Trunks product page has the full breakdown, and the team can talk through migration specifics for your setup.