SIP Trunking UK: How It Works, Costs & PBX Checks

What Is SIP Trunking and Who Needs It?

SIP trunking is a service that connects a compatible business phone system (PBX) to the public telephone network using IP-based communications. It replaces the external telephone circuits traditionally provided by ISDN or analogue business lines, while allowing a business to retain a suitable PBX, call-handling rules and supported phones.

Imagine an office with an established PBX, 28 users, a reception queue and a block of UK direct-dial numbers. The internal system may still be useful, but the older external lines are due to be retired. A compatible SIP trunk can provide the external calling connection without forcing the business to replace every extension. This is not automatic: hardware, software licences, security, numbering and technical support must be verified.

For a new business without a PBX, a managed hosted phone system may be simpler. This guide focuses on deciding whether SIP trunking is suitable, understanding the components and checking an existing platform before service approval; it is not a ranking of SIP providers.

How Does a SIP Trunk Work?

On an inbound call, a customer dials a business number. The telephone network routes it to the selected carrier, whose SIP service delivers the call to the customer’s compatible PBX. The PBX then applies its extension, department or queue rules. Outbound calls usually follow the reverse direction, subject to dial plans and caller-ID authorisation.

HOW SIP TRUNKING WORKSFrom external caller to business PBX

SIP signalling and voice media are related but distinct. SIP negotiates the call; audio usually travels using RTP or, where properly supported and configured, encrypted SRTP. A successful trunk registration does not guarantee that audio works in both directions. Firewall, NAT, codec and media-path compatibility still need testing.

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The most common procurement mistake is to assume the supplier of the trunk also manages the PBX, broadband, firewall and desk phones. That can be true for a managed offer, but only if the contract says so.

Will SIP Trunking Work With Your Current PBX?

Not every existing PBX can accept a SIP trunk directly. Some support SIP natively. Others may need additional software licences, firmware changes or a properly designed session border controller (SBC) or gateway. A very old digital PBX might be technically adaptable but uneconomic to retain.

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For 3CX specifically, its official SIP trunk requirements describe registration- and IP-based authentication, DTMF, carrier addresses and optional media encryption. Treat these as an example of vendor-level interoperability checks rather than universal rules for every PBX. For the wider PBX setup, see our 3CX phone systems guide, which separates 3CX licensing, hosting and SIP carrier connectivity.

For a Yeastar deployment, our Yeastar P-Series phone system guide explains the differences between appliance, software and cloud editions, and why the SIP carrier, numbers and support arrangement must be checked separately.

How Many SIP Trunk Channels Do You Need?

A channel generally represents capacity for one simultaneous external call through a trunk, subject to the provider’s service definition. A company with 60 staff does not necessarily require 60 channels. Conversely, a small business running busy outbound calling, reception transfers and inbound queues could need more than its headcount suggests.

Worked Example: A 24-Person Business

Suppose 24 employees use a compatible PBX. During a hypothetical busy interval there are five external inbound conversations, four outbound conversations and two additional incoming calls waiting in an externally connected queue. If those eleven external call legs each consume one carrier channel under the chosen provider’s model, the estimate is 11 active channels. A transfer or conference can introduce additional legs; verify the actual billing and routing model. The appropriate purchased capacity also needs agreed headroom.

Can You Keep Your UK Business Numbers?

Many geographic and other eligible business numbers can be transferred to a new communications provider, but porting depends on the number type, network arrangements and correct paperwork. Make a complete inventory before deciding which trunk service to order.

• Main numbers and DDI blocks: record the full numbers, active owner/provider, ranges, call destinations and any reserved numbers.
• Call routing after porting: map each inbound number to the correct PBX destination, including out-of-hours paths and department queues.
• Outbound CLI: agree which valid, diallable telephone numbers users may present; the carrier must authorise the appropriate number presentation.
• Connected services: check whether existing numbers or legacy lines also serve fax, monitored alarms, payment terminals, lift phones or broadband dependencies.
• Timing and fallback: agree porting milestones, a test number, port-failure escalation and a date when the old service can be ceased.

Ofcom sets number portability and switching obligations under its number portability guidance. Porting rights do not remove the need to establish commercial, technical and operational eligibility for a particular order.

Use our dedicated UK business number-porting guide for the wider transfer process. This SIP guide is limited to how number arrangements affect a trunk deployment.

What Does SIP Trunking Cost in the UK?

SIP trunking costs depend on the provider’s charging basis, included calls, contract term, number rental and the equipment/services required around the trunk. An advertised low channel price is not a complete business telephone quotation.

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There is no single verified “average UK SIP price” suitable for every business. For transparency, public UK procurement listings show that products can be priced by channel, user or other units. For example, a published public-sector SIP listing uses a per-licence monthly price, but that figure is not an offer for your organisation or proof of market-wide pricing. Obtain current written supplier quotes and distinguish excluding/including VAT.

Where Does SIP Trunking Fit Into the PSTN Switch-Off?

Openreach identifies 31 January 2027 as the retirement milestone for the traditional PSTN. For businesses with an eligible PBX, replacing legacy voice circuits with an approved SIP connection can be one possible transition route. It does not necessarily mean replacing the internal phone system.

It also does not mean every old device attached to a telephone line will work on a trunk or an analogue adapter. Security alarms, lift emergency communications, fax, card terminals and other non-voice devices need separate compatibility review. Some organisations should replace the PBX rather than spend money keeping an obsolete platform connected.

Our PSTN-to-VoIP migration guide owns the full business-wide migration programme. Here, the narrower decision is the readiness of the PBX-to-carrier connection.

How Do You Make SIP Trunking Reliable?

A SIP trunk is not inherently resilient because it uses IP. If the PBX, office power, router, broadband connection or carrier service fails, calls may be interrupted. Resilience has to be designed across the entire chain, with responsibilities agreed before the number port.

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A second broadband link is useful only if it genuinely avoids the relevant shared failure. An alternative SIP address does not help if both paths depend on the same single router, access circuit or unpowered PBX. Ask who can implement a network-level divert when the local PBX cannot be reached.

What About 999, 112 and Caller Location?

The SIP trunk and PBX together must support an appropriate emergency-call route. A normal outbound call succeeding is not evidence that 999/112 is correctly routed or that relevant caller-location information is accurate.

Ofcom’s General Conditions address uninterrupted access to emergency organisations and accurate, reliable location information where technically feasible. The obligations fall on regulated providers; businesses still need practical arrangements and verified information about locations, remote workers, CLI, power and alternative calling routes.

• Identify which carrier and configuration handle 999/112 for every calling site.
• Confirm the appropriate registered emergency location for the supplied numbers and how it is updated.
• Check remote or movable users, where a phone or app may be used away from the registered premises.
• Establish an alternative method for emergency calls during power or connectivity failures.
• Ask the responsible provider to explain its approved verification method; do not place unnecessary live emergency test calls.

For the detailed UK-specific requirements and a separate location-verification record, read our VoIP 999/112 emergency-calling guide. The Ofcom compliance guidance provides the relevant regulatory context.

What SIP Trunk Security Checks Matter?

SIP systems can be abused through stolen credentials, exposed services, malicious routing changes or unexpected international and premium-rate call activity. Security is shared across the PBX, network, carrier and administration process.

For the platform-specific distinction, 3CX documents support and limitations for TLS and SRTP trunking. Other PBXs and carriers may behave differently. Your approved configuration and incident response plan matter more than a general claim that SIP is secure.

SIP Trunk Compatibility and Acceptance Record

This is the practical output of the guide: a shared sign-off sheet for the business, PBX maintainer, carrier and connectivity team. Its purpose is to prevent a contract being accepted solely because one test call worked.

GO-LIVE VERIFICATIONFour Checks Before SIP Trunk Acceptance

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Decision rule: mark the deployment ACCEPT only when material controls have evidence and responsible owners agree. Otherwise record HOLD with the defect and remediation plan. This is an editorial decision framework, not a supplier certificate or a substitute for qualified engineering tests.

How to Move a Compatible PBX to SIP Trunking

1. Audit the current phone estate. Inventory numbers, sites, users, PBX software, gateways, non-voice devices, active call patterns and maintenance terms.
2. Confirm the PBX transition route. Ask the manufacturer or maintainer for a supported SIP/carrier combination and any required updates, licences or SBC.
3. Choose a carrier and define ownership. Require clear technical and commercial terms, routing, porting support, emergency arrangements, incident response and exit options.
4. Prepare network and security. Review connectivity, addressing, firewall/media routes, QoS where relevant, access controls, monitoring and power resilience.
5. Build and pilot before porting. Test a temporary number or test trunk where available, including incoming/outgoing routing, hold, DTMF, queues, transfer and both-way audio.
6. Agree cutover and rollback. Schedule number transfer, nominate decision-makers, notify users, prepare fallback instructions and identify any external dependencies.
7. Test and document acceptance. Use the acceptance record above, confirm the approved emergency verification route, then sign off defects and training.
8. Cease old services only when safe. Obtain confirmation that porting succeeded and that all dependent services have been migrated or independently resolved.

The business-wide migration sequence and dependency audit belong to our PSTN migration guide. This checklist concerns the narrower process for a specific SIP-to-PBX handover.

When Is SIP Trunking the Right Choice?

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Do not treat SIP trunking, hosted PBX and unified communications as interchangeable labels. SIP trunking is the external telephony connection; hosted VoIP is often a managed complete phone-platform service. The broader solution-selection task is covered by the published VoIP Buyer’s Guide.

Frequently Asked Questions About SIP Trunking

Official Sources and Editorial Method

This independent guide synthesises current UK network and regulatory guidance with published vendor requirements. The examples are expressly hypothetical. CompareVoIPService has not claimed hands-on SIP trunk tests or a measured sample of UK supplier prices. Technical acceptance must be performed by the appointed providers or competent installers.

1. Openreach — UK PSTN retirement and digital migration
2. Ofcom — Emergency calling access and caller location
3. Ofcom — Number portability requirements
4. Ofcom — Calling Line Identification guidance
5. 3CX — Supported SIP Trunk Requirements
6. 3CX — Configuring SIP Trunks and DDIs
7. 3CX — SIP Trunk TLS/SRTP Compatibility and Limits
8. Vodafone Business — UK SIP Service Characteristics
9. Virgin Media Business — SIP Trunking Service Overview

Editorial source review: October 2026. Prices, service coverage, SIP requirements, number portability and support terms must be rechecked at the time of a real procurement. This guide is not a regulatory certificate or carrier test report.