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Fixed lines and sip - SIP trunks

SIP trunks

With PSTN and ISDN lines being phased out by Openreach, businesses are identifying alternative solutions for their business communications. Session initiation protocol trunking or SIP trunks are one of the recommended ways forward for replacing your existing PSTN and ISDN lines, and they are highly cost-effective and flexible too.

What are SIP trunks?

SIP trunks are replacing traditional ISDN and PSTN copper phone lines services by enabling businesses to make calls over an internet connection. They can be delivered in many ways using leased lines, SoGEA or FTTP connections to provide low-cost calls, flexible numbering, and business continuity.

The connectivity and underlying infrastructure you choose to run SIP trunks over depends on several factors, such as the number of channels your business needs, local availability of services and the required service level agreement (SLA).

Some scenarios also allow for data to run over this connection, as well as voice, to reduce the total cost of ownership and the complexity of having separate voice and data connections. 

SIP trunks

Benefits of SIP trunks 

Cost reduction

It is significantly cheaper to rent 10 x SIP channels than it is to rent the equivalent in ISDN or PSTN lines. Other cost savings are realised through free calls to UK landlines and mobiles, and savings achieved by companies using their internet connection for both voice and data.

Business continuity

In the event of a line failure, it can take up to 24 hours to activate a re-direct on an ISDN channel. A re-direct on SIP trunks takes just minutes. ISDN redirect services are also limited to a single number whereas SIP can re-direct to up to 150 direct dial-ins (DDIs), including mobile numbers.


Adding ISDN channels or extra services to can be time consuming and will not be possible from the end of 2023 as Openreach continues to withdraw them ahead of the 2025 switch-off deadline. SIP trunks represent a much more flexible approach to channel control and diverts are almost instantaneous.

Geographic presence

You can choose which numbers your customers call you on and  which number is displayed when you call them to provide a geographic presence in any area code you desire, even if you do not have an office there. This will give you the opportunity to market to a locality that you wish to break into.

Protection from telecoms fraud

Our SIP partner Gamma, offers a free fraud mamagewment service whereby your call activity is monitored to look for call profiles that do not fit your business. When we detect such activity, we take action to contact you and alert you to the situation. If we are unable to contact you because the fraud is happening over a weekend, for example, we can put measures in place to cease the calls. In fact, we are so confident in our ability to detect abuse that we limit customers’ liability to £750 for any instance of fraudulent activity.

Does my business need SIP trunks?

If you are still using PSTN or ISDN lines, then SIP trunks could be what you are looking for. PSTN and ISDN are on their way out as part of Openreach’s copper withdrawal programme. You must replace these traditional phones lines by either switching to SIP trunking or to a cloud-hosted phone system.

As the Openreach switch-off will affect every business that is still using PSTN and ISDN lines, waiting until the switch-off date at the end of 2025 is not a sensible course of action. Businesses need act now to make sure they have effective business communications in place for the future.   

Frequently asked questions

1. Will my supplier just switch me from my PSTN or ISDN to SIP?

No. They may have contacted you about this, but you will need to seek out a SIP provider or choose a replacement phone system. This is not the time to put your head in the sand! You need to make a plan and we can help you.

2. Is my phone system compatible with SIP trunks? 

If your phone system is rather old and the manufacturer has already declared it end of life, then probably not. Let us know what system you are currently using, and we can advise you. Some phone systems can be upgraded to SIP with a software upgrade. 

3. What’s the difference between VoIP and SIP? 

VoIP stands for voice over internet protocol and is used to describe a type of phone system that allows calls to be made over internet connections instead of using traditional phone lines. VoIP describes the method of making calls, so essentially you can turn an existing on-premise phone system into a VoIP solution by using SIP trunks. 

Useful resources

> Are you searching for replacement parts on the internet for your obsolete telephone system?

ISDN to SIP guide

Out with the old, in with the new: The ISDN to SIP migration process

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