Enterprise SIP Trunking
Reliable, scalable SIP trunk services designed for organisations that depend on voice as a critical business channel.
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In short
SIP trunking connects your phone system (an on-premise PBX or a cloud contact-centre platform) to the public telephone network over a data connection instead of ISDN lines. DigiTalker provisions the trunks, ports your existing numbers and integrates them with your phone system, with automatic failover across multiple carrier paths.
Managed SIP Trunk Provisioning
Our SIP trunking service provides enterprise-grade voice connectivity through managed carrier relationships. We handle the full provisioning lifecycle from initial capacity planning through to live deployment, including DDI allocation, number porting, and integration with your existing PBX or contact centre platform. Each deployment is configured with automatic failover routing across multiple carrier paths, so that inbound and outbound calls continue uninterrupted in the event of a circuit or carrier issue.
We include fraud monitoring as standard across all SIP trunk deployments. Our systems analyse call patterns in real time and apply automated controls to prevent toll fraud, international revenue share fraud, and other common attack vectors. Clients receive alerts and can set custom thresholds that match their expected traffic profiles.
Call capacity is managed proactively. We monitor concurrent call volumes against provisioned capacity and recommend adjustments before limits are reached. This ensures that peak periods, campaign launches, and seasonal volume increases do not result in blocked or failed calls. Detailed call records are available through our reporting portal, giving full visibility of traffic volumes, call durations, and routing outcomes.
Key Features
- Resilient SIP delivery
- Geographic & non-geographic numbers
- Automatic failover
- Fraud detection & prevention
- Call capacity management
- Detailed call records
What is SIP trunking?
SIP trunking is a way of delivering telephone calls over an internet or private data connection instead of traditional ISDN phone lines. A SIP (Session Initiation Protocol) trunk connects your phone system (an on-premise PBX or a cloud contact-centre platform) to the public telephone network through a provider, carrying multiple simultaneous calls (channels) over a single connection. Because it is software-defined, capacity can be scaled up or down without installing physical lines.
How SIP trunking works
When someone calls your business, the call is routed over the SIP trunk from the public network to your phone system, and outbound calls travel the same path in reverse. Each trunk carries a defined number of channels (one channel per simultaneous call), and calls are set up and cleared using the SIP signalling protocol. Voice is carried as data packets, so quality depends on sufficient bandwidth and properly configured quality-of-service (QoS) on your network, which we assess and monitor as part of the service.
SIP trunking and the PSTN ‘big switch-off’
The UK’s traditional analogue PSTN and ISDN network is being progressively withdrawn ahead of the national ‘big switch-off’, which Openreach has targeted for the end of January 2027. Any organisation still running voice over ISDN needs a migration plan before then. SIP trunking is the direct, like-for-like replacement: it keeps your existing numbers and phone system where possible while moving the underlying connectivity to IP. Migrating early avoids a last-minute rush and unlocks the cost and flexibility benefits sooner. Our PSTN switch-off guide sets out the steps.
Benefits of SIP trunking for UK businesses
- Lower cost: no physical line rental and reduced call charges, particularly for multi-site and international calling.
- Scalability: add or remove channels in software as call volumes change, with no engineer visits or new line installations.
- Resilience: automatic failover across multiple carrier paths keeps calls flowing if a circuit or carrier has a problem.
- Number flexibility: keep your existing numbers and add geographic, non-geographic (0800 / 03) or international numbers on demand.
- Future-proof: a clear path off ISDN ahead of the PSTN switch-off.
SIP trunking FAQs
How much does SIP trunking cost in the UK?
SIP trunking is usually priced per channel (each channel carries one concurrent call) plus call charges, and is typically significantly cheaper than equivalent ISDN lines and line rental. The right number of channels depends on your peak concurrent call volume, so we provide a tailored quote based on your capacity and number requirements rather than a fixed list price.
How many SIP channels do I need?
One channel supports one simultaneous call. As a rule of thumb, size your channels to your busiest-hour concurrent call volume rather than your total number of users, because not everyone is on a call at the same time. We help model this from your call data during provisioning.
Can I keep my existing phone numbers with SIP trunking?
Yes. We port your existing geographic and non-geographic numbers to the new SIP service so customers reach you on the same numbers, and we manage the porting process end to end.
What happens if my internet connection fails?
Our SIP trunks are configured with automatic failover routing across multiple carrier paths, and calls can be diverted to alternative destinations such as mobiles or another site if your primary connection is unavailable, so you don't miss calls.
Do we need SIP trunking before the PSTN switch-off?
If your phone system still runs on ISDN, you need SIP trunking or another IP-based service. Openreach says all traditional phone lines will be going digital by 31 January 2027, and BT Business says ISDN services stop working from that date if they have not been moved. SIP trunking is the direct replacement for ISDN and keeps your existing numbers.
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Discuss Your SIP Trunking Requirements
Our voice engineering team can assess your current connectivity and recommend a SIP trunking configuration that meets your capacity, resilience, and compliance needs.
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