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SIP Lines

SIP Trunking: a flexible, scalable ISDN replacement from Immervox

ISDN lines are going to be inoperable by 2025 due to the PSTN switch-over and WLR withdrawal. It’s estimated that 2 million UK businesses still rely on ISDN networks and lines. Since the PSTN was announced we’ve helped hundreds of customers replace their soon-to-be obsolete ISDN lines, and for many SIP lines have been the flexible, scalable alternative they never knew they needed.

What are SIP lines?

The ISDN (Integrated Services Digital Network) phaseout and shut-down is in full swing. From 2023 BT are no longer going to be supplying new ISDN connections, and as of 2025 the PSTN that ISDN connections rely on will be fully switched off.

This has left many of our customers needing a suitable replacement to the ISDN lines they’ve relied on for years. Whilst many alternatives are available, SIP lines have proven themselves as a flexible, scalable alternative to ISDN that brings increased functionality without compromising on call quality or pricing. 

SIP Trunking is a voice, video and conferencing connectivity solution. There is a reason SIP Trunking and SIP lines are becoming incredibly popular amongst businesses worldwide. SIP Trunking is an API that allows your business to transfer voice and data across the same channel. 

ISDN delivers voice data across a traditional copper telephone network. In the UK, the national copper network is in the process of being replaced by a national fibre infrastructure. Replacing your ISDN lines with SIP ensures your phone systems isn’t put out of action by the PSTN switchover and provides a superior service at a much more streamlined cost.  

The benefits of SIP Trunking

Your businesses ISDN network has provided a reliable, secure connection for years. Unfortunately like all technologies ISDN has reached the end of its life cycle in the UK. SIP Trunking is the powerful, scalable, and cost-effective replacement hundred of UK businesses are choosing as their new voice and connectivity solution. Here are just some of the reasons so many Immervox customers are so positive about their new SIP lines.


As an IP based voice solution, SIP capacity can be upscaled or reduced to match your varying demands. Extensions and users can be added quickly and easily, and configuration can be managed from the comfort of your browser. 


SIP lines can carry voice and data. A SIP network allows for voice and video calls across the same connection, saving immensely on your monthly communications expenses. As your SIP provider, Immervox will ensure your service always fits the needs of your business. 

Superior Call Quality

SIP lines guarantee optimum call quality. With SIP Trunking you can relax knowing that your staff can be heard and understood during every crucial customer and client phone call. 

Easy To Install

SIP Trunking requires no additional hardware infrastructure, and can be integrated with your existing data network. This means that installation and deployment is quick and usually challenge-free. What’s more, having no costly hardware investments saves on long term maintenance expenses.

Optimised Costs

On traditional systems your telephone voice services were costed and billed separately to your broadband and data. By consolidating voice and data onto a single network, SIP Trunking makes all communications and connectivity expenses part of a single, manageable monthly service package. 

Future-Proof Technology

SIP is an IP based voice solution. This means that it carries calls over your existing broadband connection. This also means that SIP Trunking is fully compatible with cutting edge Cloud, VoIP, and Unified Communications solutions. SIP is a future-proof technology that ensures your voice infrastructure is never a barrier between your business and whatever industry-redefining technology awaits in the future. 

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