SIP Trunking

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What is SIP Trunking for Business?

When you make a phone call from your office, your phone talks to a mini telephone exchange called a PBX. Your PBX system then talks to the public telephone network, connecting to it via a 'trunk'.

As the name suggests, trunks are far wider than normal phone lines. In fact they typically have 2 to 30 times the capacity of a standard phone line.

Most trunks in the UK are ISDN circuits. But there's a new kid on the block called SIP that's gaining a lot of attention. SIP stands for Session Initiation Protocol. It is designed to set up connections between devices, such as computers, phones, PBXs, video conferencing units and servers.

SIP can be used to support a wide range of services, including video broadcasts, instant messaging, multi-player gaming and telephone conferences. But the application we're interested in is setting up virtual phone lines between your PBX and the public telephone network. This lets calls travel over your internet connection, and so reduces or eliminates the need for expensive ISDN circuits.

Many recently manufactured PBXs support SIP. If your PBX doesn't, don't worry. You can still benefit from SIP, by adding a SIP-ISDN Gateway. This is a box that converts the ISDN signals from your PBX into SIP. Trunk signals, and vice versa.

SIP Illustration

Is SIP Relevant To Your Business?

SIP is relevant to you if:

SIP is not relevant if:

SIP Trunking Benefits

SIP trunking offers a lot of benefits relative to ISDN:

The Risk-Free Hybrid Solution

If you're convinced that SIP offers benefits, but would prefer to 'dip a toe in the water' before getting rid of your ISDN lines, there's a hybrid solution that will appeal to you. Here's how it works.

Your SIP Trunk provider would put a box between your PBX and your ISDN lines.

All incoming calls to your office would go over ISDN, just as they do now.

All outgoing calls would be re-routed by the box, to go over your Internet connection, using SIP trunks. In the event that your Internet connection were to go down, the calls would go out over your ISDN lines, just as they do now.

The result is you get cheaper call rates (because the calls you make go out via SIP rather than ISDN).

You can reduce the number of ISDN channels you pay for, if you wish, but still have some left, just in case they're needed.

The right set-up to ensure reliability involves:

Voice Prioritisation - the SIP trunks should take priority over the rest of the traffic, as phone calls aren't delay-tolerant or loss-tolerant.

Automatic 'fail-over' to backup connection - there must be at least one backup-up connection. This can be a backup internet circuit, set to automatically take over if the main link goes down. Or it can be a left-over ISDN circuit (with fewer channels than before).

Direct connectivity to your SIP trunk provider - your SIP trunks and Internet connection should be from the same company, to ensure your phone call quality is good. Although we talk of your 'Internet connection', in reality you only want your calls to travel over the dedicated, uncongested bit of the Internet that you control, so that traffic congestion doesn't ruin your call-quality.