Cheap calls provider setup

Hi there!

I'm new to this forum and basically to Trixbox so please be patient (I'm sure you all are!)

We use Trixbox Version: 2.6.0.0 with Asterisk 1.4.18.1-2 RPM

The server uses an E1 connected to a Tiger Jet Network Inc. Tiger3XX Modem/ISDN interface

We have 30 channels on this E1 and several telephone numbers assigned by our carrier to this line.

There is a specific telephone number (and only that number) that we need to configure so it always calls out through a cheap calls provider by dialing a prefix.

Example given:

The number in question is 952xxxxxx and everytime someone calls using an extension configured with that number as its CID NUM Alias, trixbox should add the prefix 10xxx for every call, not affecting the rest of the numbers we use at the office for outgoing calls.

We use the CID NUM Alias so that the extension that calls always gives the right number of the office it's calling from so anyone (clients, etc) can call back to the right number (where that extension is)

I hope to have explained myself properly and that some mercyful soul will give me a hand on this.

Thanks in advance :)