Asterisk

I'm currently testing a new system based on an Asterisk / Trixbox server we set up in London, except the 'Voice Over IP' system is currently lacking in 'Voice'. Once it's live I'll likely hire an Asterisk genius, but for now I just want to get a basic handle on the system myself.

The inbound and outbound trunks are all installed and work perfectly, so that's not the issue, I can make calls in and out, and I managed to make the PBX redirect an incoming DID to my mobile, that worked also.

The problem I'm having is with no sound being able to be heard calling between internal extensions, at the moment I'm just testing with the X-Lite softphone software, and every time I make a call I can answer the call, put it on hold (which works), etc, just no sound at all.