What is a SIP Trunk

Session Initiation Protocol (SIPTRUNK is a cost saving service that uses the VoIP (Voice Over IP) protocol to provide connectivity between a phone system and the PSTN (Public Switched Telephone Network).

SIP Trunk is the future of business communications and is commonly referred to as the replacement to ISDN.

Future-proofing with cloud telephony

Businesses of all sizes should be starting to plan for the modernisation of the telephone network and thinking about moving their communications to the cloud to stay ahead of the coming changes.

Your voice and data connections are vital components of your business – but so are the costs of running multiple lines, their flexibility and how much control you have over them.

Increasingly Arrow customers are taking VoIP based services, either one of our hosted telephony products, Scala or Horizon, or using SIP trunks on their phone system.

SIP Trunks allow greater control and flexibility over your business telephony. They are also a low-cost alternative for ISDN services which will be switched off by BT in 2025 and unavailable to buy after 2023. Learn more about the ISDN Switch-off.

Reliable voice services

SIP trunking is rapidly replacing ISDN services by connecting your site directly into the Gamma network (Arrow’s chosen partner) via an IP connection to carry and terminate your inbound and outbound voice calls across the public telephone network. This is an end-to-end service with an availability guarantee, voice channel guarantees and voice Quality of Service.

Gamma SIP Trunks provide a highly-flexible alternative to ISDN and are compatible with all the leading IP PBX brands in the UK market, giving you peace of mind that your PBX hardware works with the network service. Compared to ISDN, SIP Trunks are cheaper on a per-channel basis and more flexible in terms of what telephone numbers you can have and the locations in which you can have them. In addition, they are quicker to install and offer a robust business continuity service that ensures your business never loses any calls. Our service can support connections ranging from two channels for businesses with small PBXs, with no limit on the number of channels that can be supported for large enterprises and contact centres.

With Gamma you get a service that is more robust and cost-effective than ISDN. Because SIP trunking is an IP-based telephony service, you also get number flexibility, which gives you a whole host of other added benefits that can solve real business issues. Here are just some of the other ways that SIP Trunks can enhance your business:

Arrow can also offer International SIP trunks to enable our customers to fully integrate their sites.

Business continuity

Use Gamma SIP Trunks to protect your office environment in case of fire, flood or other natural disaster. For example, if your London branch has to close in an emergency and you need calls routed to Manchester immediately, this can be done at no extra charge to you. Set up at your new location is quick and easy and there are no call forwarding costs for each inbound call.

Feature rich, centralised SIP Trunk Call Manager

SIP Trunk Call Manager provides you with all the features and benefits of Gamma SIP Trunks together with feature rich, centralised call control. This includes our market leading call bundles and the only fraud management tool that protects you and your customers by enabling them to set spend limits. SIP Trunk Call Manager offers a powerful business continuity solution, giving you the ability to manage your number estate and all aspects of your inbound calls. Consolidate all your numbers onto a single platform and never miss a call!

SIP Trunk Call Manager provides you with all the benefits of Arrow’s SIP together with a feature-rich, centralised inbound call management service. Continuity with Call Manager means you have the ability to manage your entire number estate and all aspects of your inbound calls.

together with a feature-rich, centralised inbound call management service. Continuity with Call Manager means you have the ability to manage your entire number estate and all aspects of your inbound calls.

If you’re looking for a solution that offers powerful business continuity and the ability to fully manage your entire number estate directly, SIP Trunk Call Manager is the ideal solution. 

Click here to learn more about our Data Connectivity services.

Legacy ISDN technology might be holding back your business

Technologies like SIP trunking offer a more flexible, cost-effective telecoms solution for your organisation to help you achieve your business goals. That is why it’s time to add ISDN replacement to your business agenda.

Cheaper than ISDN

Companies using Arrow’s SIP connections will be charged at considerably lower rates than BT’s on all UK fixed, mobile and international Calls. In general, all SIP trunk charges will be less than what you currently pay your present ISDN supplier.

Rationalise your lines

SIP Trunks enable businesses with multiple sites to decrease the number of lines and PBX systems deployed across their organisation.

Easy Implementation

Installation is simple and non-disruptive. Suitable for any sized organisation from small businesses to large enterprises. Arrow’s SIP Trunk service is compatible with most of the UK’s IP PBX manufacturers. Arrow can advise on upgrading equipment, leased lines and Ethernet products to guarantee the best results from the SIP Trunk service.


Gamma SIP Trunks are valuable for businesses with seasonal voice capacity requirements where more lines need to be quickly and easily implemented with no long-term commitments, for example when running sales campaigns or during major sports or entertainment events. Unlike ISDN, SIP trunks are a highly cost-effective system that can be scaled up and down according to the requirement, enabling more effective use of IT team resources by making the provisioning of new users with voice and data systems quick and easy.

Number flexibility

SIP Trunks allow you to retain local numbers even if your office is located elsewhere.

Do you need expert advice? Contact Us.

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With over 20 years in the business telecoms industry and an unrivalled reputation of delivering excellent, personal customer service, Arrow is one of very few companies in the UK able to provide a full telecoms, IT and energy consultancy and service proposition.

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