Least cost routing

As well as connecting to the PSTN directly, the local PBX can make and receive calls via external VoIP gateways. This can help reduce costs, especially when calling long distance.

To do this you need to subscribe to a public service provided by an Internet Telephony Service Provider (a list of providers is available at www.itspa.org.uk). Most offer Session Initiation Protocol (SIP) services which effectively give you one or more virtual (VoIP) telephone lines. They can either be mapped to your own telephone numbers (check with the provider for DDI support) or new numbers which, in most cases, can be on any UK exchange or even international codes.

The PBX software can intelligently route calls over ISDN or virtual SIP lines depending on the dialling code, the caller's identity and other parameters.