SIP Trunking Explained: What It Is and How It Works

What a SIP trunk actually is

Session Initiation Protocol, usually shortened to SIP, is the signalling standard that sets up, manages and ends voice calls across an internet protocol network. A SIP trunk is the virtual equivalent of the physical phone lines a business once rented: instead of copper pairs running into the building, the trunk is a data session that carries voice traffic between the organisation's telephone system and the provider's network, which in turn connects to the wider public telephone network.

The practical effect is that calls travel as data packets over a broadband or dedicated internet circuit rather than over dedicated voice wiring. Because the trunk is software-defined, capacity can be adjusted by changing the number of concurrent channels rather than by ordering or removing physical lines. This flexibility is one of the main reasons businesses move to SIP as the underlying copper network is withdrawn.

It helps to separate the two jobs SIP performs on a call. The signalling layer sets up the call, negotiates which codec both ends will use, rings the destination and tears the call down at the end. The media layer, carried by a companion protocol, transports the actual audio packets once the call is connected. Understanding that split matters in practice, because most quality and connectivity problems on a SIP trunk trace back to one layer or the other: signalling faults stop calls connecting, while media faults let a call connect but degrade the sound. A provider's session border controller sits at the boundary to manage both, registering the trunk, authenticating it and shielding the business PBX from unwanted traffic arriving from the public internet.

How SIP replaces ISDN lines

Integrated Services Digital Network, or ISDN, was for decades the standard way that businesses connected a private branch exchange to the outside world. ISDN2 delivered pairs of channels and ISDN30 delivered blocks of up to thirty, each channel carrying one call. Openreach has confirmed it has stopped selling new ISDN and analogue Wholesale Line Rental products, and the underlying public switched telephone network is being withdrawn as part of the all-IP migration completing in 2027.

SIP trunking is the like-for-like replacement for that connectivity. Where a business previously rented thirty ISDN channels, it can provision a SIP trunk with an equivalent number of concurrent channels delivered over its internet connection. The PBX continues to manage internal extensions and call routing, but the external leg is now carried over IP. Geographic numbers can usually be ported across, so customers continue to dial the same number after the change.

The migration mechanism is worth understanding because it shapes how a business should plan the move. An organisation can keep its existing on-premises PBX and simply swap the ISDN circuit for a SIP trunk, provided the PBX speaks IP or can be fitted with a gateway that translates between the older system and the trunk. That route preserves the investment in handsets and call-flow configuration while removing the dependency on copper. The alternative is to replace the PBX entirely with a cloud-hosted system, which changes more at once but removes on-site hardware. Either way, the prudent step is to run the new service in parallel and test it before the legacy circuit is ceased, so that any number-porting or configuration issue surfaces while the old line is still available as a fallback.

How many channels a business needs

Sizing a SIP trunk is about concurrency, not headcount. A single channel supports one simultaneous call in either direction, so the question is how many calls the business expects to be active at the same time during its busiest period. An office with forty staff might never have more than ten calls running concurrently, in which case ten channels would meet demand with a margin to spare.

Call centres and customer-facing teams behave differently, because a high proportion of staff may be on calls at once. A common approach is to review historical call records for peak concurrency, add a buffer for growth and seasonal spikes, then provision channels accordingly. Because SIP capacity is software-defined, channels can often be added quickly when demand rises, which removes the long lead times associated with ordering physical ISDN circuits.

There is a useful cross-check between channel count and bandwidth. Each active channel consumes a defined amount of upstream and downstream capacity for the duration of a call, and that figure depends on the codec in use, so the trunk must be matched to a connection that can carry every channel at peak without starving the rest of the network. A trunk provisioned for thirty channels on a circuit that can only comfortably carry ten concurrent calls will sound fine when quiet and fall apart at the busy hour. Sizing the channels and sizing the connectivity are therefore the same exercise viewed from two angles, and both should be set against the genuine peak rather than an average.

SIP trunk versus ISDN at a glance

The table below sets out the main practical differences between a SIP trunk and a traditional ISDN connection, covering how each is delivered, scaled and maintained.

The cost model and how to choose a provider

SIP trunk pricing is typically built from a per-channel rental, a charge for the internet connectivity that carries the traffic, and call charges that may be bundled into inclusive minute packages or billed per minute. Because there is no per-line copper rental, the recurring cost structure differs from ISDN, and businesses with fluctuating volumes often value the ability to flex channels rather than pay for permanent capacity that sits idle.

When assessing a provider, the practical points to examine include the underlying network the calls traverse, the call quality commitments set out in any service level agreement, support for number porting, and the security controls protecting the trunk. Ofcom rules on number portability mean an existing geographic number can usually move across, but confirming the porting process and timescales in advance avoids surprises during cut-over. It is also worth establishing how the provider authenticates the trunk and guards against toll fraud, because an exposed or weakly secured SIP service can be exploited to place expensive calls, and a sensible contract sets out call-spend caps and alerting to contain that risk.

Resilience options worth planning for

Because SIP traffic depends on the internet connection, resilience planning matters more than it did with a dedicated voice circuit. Common arrangements include automatic failover that reroutes inbound calls to an alternative site, to mobile devices, or to voicemail if the primary connection drops, so customers are not met with dead air during an outage.

Businesses that treat voice as critical often pair a primary circuit with a diverse backup connection and configure call routing rules that redistribute traffic without manual intervention. It is also worth confirming how the provider handles emergency calls, since the way a 999 call is located on an IP service differs from a fixed copper line and should be documented during set-up. Providers are expected to pass an accurate registered address so that emergency operators can route the call and dispatch help to the right place, and a business that moves a SIP-connected device between sites should update that address rather than assume it follows the handset automatically.

Security and fraud protection on a SIP trunk

Putting voice on an IP connection means the same connection can, if left open, be probed from the public internet, so security is part of running a SIP trunk rather than an optional extra. The session border controller is the first line of defence: it registers and authenticates the trunk, rejects traffic from sources that are not recognised, and can rate-limit attempts to flood the service. Strong credentials on every device, prompt patching of the PBX and gateway, and restricting which networks can reach the trunk all reduce the surface that an attacker can reach.

The financial risk that most often catches businesses out is toll fraud, where a compromised system is used to dial premium or international destinations and run up a large bill before anyone notices. Practical controls include barring call types the business never uses, setting a ceiling on daily call spend, and switching on alerts that flag unusual patterns such as a sudden surge of overnight international calls. Because handling call detail and recordings also touches data-protection duties, organisations should keep that processing proportionate and documented in line with general UK data-protection principles, treating call records with the same care as other personal data.

Frequently Asked Questions

What is a SIP trunk?

A SIP trunk is a virtual phone line that carries voice calls over an internet protocol connection between a business telephone system and the public phone network. It performs the same job as a traditional ISDN line but is delivered as a data session rather than over dedicated copper. Capacity is measured in concurrent channels rather than physical lines, and signalling and media travel as separate flows.

How many SIP channels does my business need?

The number of channels is set by peak concurrent calls, not by the number of staff or handsets. Each channel supports one simultaneous call, so a business should review its busiest call periods and add a buffer for growth. Because channels are software-defined, capacity can often be increased quickly if demand rises, but the carrying connection must be sized to match the channel count.

How does SIP trunking work with my existing PBX?

A SIP trunk connects to an existing PBX through a session border controller or an IP-capable gateway, depending on whether the PBX already supports IP. The PBX continues to manage internal extensions and routing while the external leg travels over the trunk. Older analogue or digital systems may need an adaptor to bridge to the IP connection, and running the trunk in parallel before ceasing the old circuit is the safest way to migrate.

Is SIP trunking more reliable than ISDN?

SIP reliability depends on the quality and resilience of the internet connection carrying the traffic, whereas ISDN ran over dedicated voice circuits. With a well-provisioned connection and failover routing, SIP can offer resilience that ISDN could not, such as automatically rerouting calls to other sites or mobiles. Without that planning, an internet outage can interrupt service, so a diverse backup connection is worth considering.

How much does SIP trunking cost?

SIP pricing usually combines a per-channel rental, the cost of the connectivity carrying the calls, and call charges that may be bundled into inclusive packages or billed per minute. There is no per-line copper rental, and channels can often be flexed up or down. Exact figures vary by provider, call volume and connectivity, so quotes should be compared on a like-for-like basis, including any fraud-protection and porting terms.