Dialplan for a specific Extension

Hello,

I have an Asterisk PBX running. I also have a few SIP lines (SIP trunks) registered and configured.

What I want to do is that all the outgoing calls from a specific extension, going out for a specific SIP line (and not for through the other lines).

I don’t know exactly if I can configure it directly in the FreePBX web management or if I have to edit some file (and in this case I don’t know what is the exactly file that I have to edit and how).

I will be very gratefull for every help.

Thanks in advance.