Sorry cosy,
I will do one up and send it soon.
http://www.astrasip.com.au I found was good ... no monthly fee
10c calls flat rate all around Australia and 29c/min mobile calls.
I also found that the call quality was very good ... I have my parents and one of my friends on them as well.
I don't know much about the billion router.
I have my SPA3000 connected to my asterisk now ... working well.
Asterisk is basically a PABX in the sense that it can do what the businesses have.
You can setup the digital receptionist to pick up the call and ask what extension they would like to be connected to etc as well as queues conference calls etc.
I even have a soft phone setup at work that connects into my asterisk box so I can call my wife at home or she can call me at work for free

I'll send you a diagram of what I used to have setup ... and my setup now.
I did a sketch but it was in OpenOffice so I though it would be useless to you unless you load OpenOffice on your machine .... I run Linux at home not windows .... and didn't have a copy of MS office at home.
I purchased a copy of MS office on Monday mainly for the kids for school work etc so I can do a sketch in office for you now.
Regards,
Tib.