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VoIP and SIP – which ISDN alternative is best for your business?

VoIP and SIP – which ISDN alternative is best for your business?

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The ISDN switch-off is coming, which means copper-based telephone lines are being phased out, before being completely switched off in 2025.

While ISDN and PSTN were groundbreaking at the time, these have since been superseded by new technologies – ones that can deliver the volume and quality of voice data needed today.

The two main alternatives for ISDN are VoIP and SIP – here’s more about both and why they might benefit your business.

VoIP (Voice Over Internet Protocol)

VoIP is probably the most well-known and popular telephone system for businesses looking to upgrade from ISDN.

It’s an excellent alternative for a number of reasons. Firstly, it gives you a great service without any of the on-premise hardware required by traditional telephony and it’s incredibly easy to scale up or down as your company grows or needs to meet demand. There is also no geographical restrictions which means your employees can have the same user experience no matter where they are in the world – on any device.

We cover how to switch to VoIP and its benefits in our blog. To find the right hosted VoIP solution for your business get in touch with our team here.


SIP or Session Initiation Protocol is a hosted solution for a telephone system.

It connects the PBX (your business phone system) to the network via either broadband, ethernet or a private circuit and it goes beyond voice calling by allowing you to exchange messages, video, files and other forms of data over the internet.

SIP trunking is tech jargon for a system which provides network access to many people by sharing a set of lines. A ‘trunk’ is one communication channel between one point and another, typically one being your PBX.

SIP trunks are cheaper to use, easy to install and offer greater flexibility over phone numbers. Any business with a compatible PBX can switch to SIP trunking and immediately benefit from a flexible, cost efficient and more resilient phone service.

If you’re looking at making the transition from ISDN to SIP, read more detail in our blog – ISDN is out, SIP trunking is in or download our guide.

What are the main differences between VoIP and SIP?

VoIP is limited to carrying voice data (phone calls) over the internet, whereas a SIP trunk can transfer voice, text or video. A SIP ‘trunk’ only differs from a SIP phone in that it’s expected to carry more than one VoIP stream at once.

VoIP or Voice Over Internet Protocol makes and receives phone calls over the internet or internal networks, whereas SIP is an application layer protocol that makes, manages, and terminates multimedia sessions, including voice, video, and messaging. While SIP is an initiation, it’s mainly used to support IP technology, like VoIP calls.

If your business uses ISDN, you should start to think about moving to a new service in preparation for the switch-off. In our OneTALKS webinar: 'Preparing for the ISDN/PSTN Switch Off - Your Questions Answered', we tackle the frequently asked questions we receive around the ISDN/PSTN switch-off, how to prepare and minimise the impact of this change, plus practical guidance on alternative telephony solutions to suit every budget.

Contact our team at Onecom today on 03333 44 55 01 today or get in touch.