F One SIP Trunks

F One SIP Trunks
SIP Trunking from the UK's leading provider*

F One SIP Trunks connect your business with our nationwide network, gives access to the PSTN and provides a more flexible and lower cost alternative to ISDN for inbound and outbound voice calls.

What is SIP Trunking?

SIP Trunking is our standards based replacement for traditional ISDN. It connects your PBX to the PSTN via broadband, Ethernet or private circuit into our national network. Our SIP Trunks work seamlessly with each of the leading IP PBX brands in the UK market.

Solutions for your business
Number flexibility

F One SIP Trunks support business relocations by enabling a geographic number to be retained without call forwarding costs. This ensures continuity of business and reduces the costs of having to change company stationery, listings and advertisements. As a leading UK communications provider we can also supply out-of-area geographic number ranges to make your organisation appear local, even without a physical presence.

Business-grade voice and internet convergence

With our SIP Trunks delivered over F One Broadband or F One Ethernet, you'll have a high-quality voice and data service from just one connection, making for significant cost savings.

Line and PBX estate rationalisation

For businesses with several locations our SIP trunking service provides ISDN line rationalisation - the ideal way to reduce the number of PBXs requiring maintainence.

Business continuity

Businesses of all sizes need a business continuity plan for their telephony. Our SIP trunks, when combined with our Inbound service, means your callers can always get through. Simply divert published numbers to backup locations or mobile phone - instantly and from anywhere.

Affordable resilience

If you've ever had to deal with a service outage you'll know first hand that speed of response is key. Our active/standby and load sharing designs are engineered to cope with any situation so maintaining telephony uptime. Whether you need to keep your businesses running in an emergency without the high cost of call forwarding or simply need to balance call loadings between sites during peak, F One SIP Trunks will provide a business grade solution.

Free UK calls and fraud protection

Our SIP trunking service provides free call termination to UK fixed destinations, including 01, 02 and 03 number ranges. In addition, SIP provides free calls between connected sites, including international locations. CallGuard protection from unusual usage is also included free of charge with all of our SIP trunks.

The benefits of F One SIP Trunks

Compared to ISDN our SIP trunking service is less costly per channel and more flexible in how and where geographic telephone numbers can be used. It's also quicker to install and provision and, as standard, it delivers extremely robust business continuity.

The service can support connections ranging from two channels for small PBX-equipped businesses to an unlimited number of channels for large enterprises and contact centres.

Save money on call costs and line rental

SIP delivers less costly, more reliable connectivity than ISDN over an IP connection. It can save up to 50% on line rentals and 25% or more on call costs.

Free internal calls and great features

SIP delivers less costly, more reliable connectivity than ISDN over an IP connection. It can save up to 50% on line rentals and 25% or more on call costs.

Scale up and down when you want

SIP trunks are ideal if you are moving, upsizing or downsizing and want to keep your existing numbers. Scale up lines during busy periods and back down again afterwards. You can add lines, take them away, split calls at any time. You are always in control.

Built-in Business Continuity

SIP trunks can handle emergencies easily. They are inherently disaster recovery / business continuity ready, letting you reroute calls to an alternative location quickly and easily.

My business issues

Here are just some of the current business issues we encounter with our customers where we can really help.

Business Continuity

Did you know that Green Flag Breakdown research found that one week of bad weather in 2013 was estimated to have cost UK businesses over £318million and 51 million working hours in lost productivity because business continuity plans hadn't been put in place? Scary statistics. But should disaster happen, you can use SIP trunking to minimise disruption to your business. For example, if your London branch has to close in an emergency, and you need the calls routed to Manchester immediately, F One SIP Trunks can do this and at no extra charge to you.

Multi-site line rationalisation

If your business has multiple sites with ISDN connections into each, using F One SIP Trunks you can reduce the number of ISDN connections that your business requires and also reduce the maintenance costs on all of your various PBXs.

At the remote sites you are left with just the cost of an IP connection and the handsets that you need and you can then pull all the telephony traffic back into your centralised PBX.


SIP trunks are valuable for businesses with seasonal voice capacity requirements where more lines can be quickly and easily implemented with no long term commitments, for example when running sales campaigns.

Moving office

Number flexibility with SIP trunking means that you can keep your existing number, even when moving out of the area, saving you costs on printed company literature and stationery whilst maintaining the perception that you still have a presence in the area you've moved from.

We can easily transfer your old or existing numbers too so that you can avoid expensive Remote Call Forwarding (RCF) or call diversion charges.

Flexible number display

Decide which number you want to display on a call by call basis! Great for contact centres. Use CLI flexibility to present your preferred number on your outbound SIP trunk calls.