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….
… as users of the product.