Stay away from Advanced VOIP billing, unless you like pain and suffering.
The support of the product is a pile of ***p,they do not even answer the phones during the Muslim holidays.
They claim that they have technicians on the US, but really answer the phones in Pakistan.
I use Data Techs Labs on my Quintum, with moderate loading and so far no mayor troubles.
Voicemaster???? Be sure that you talk with people that neededed support from them.