If I was you, I would not listen to Mr. “VoIP” recommending only one solution (the one of his own company).
If you see how many pakistani people (Oh boy, they are really a lot) crying desperately for help on the forum, asking stupid questions, just think how the same people have created a good (with all the things that we call “good”) billing software…
Real professionals will suggest you at least two possible vendors as a solution. Based on their own experience….
Sam, I will not be using advancedvoip if I was you.
I bought and paid for that software diecttly from the people that claim that they wrote it.
The system did not work, they have basic troubles that they could not fix, the customer service is a joke and they a bunc of very unprofessional people.
Unless you like to expend countless hours on IM, only to repeat the explanation of your trubles over and over to people that do not care, stay away from those guys.