your work is quite fine for me, yet think adding A2billing would make it near to perfection.
Many companies might use your system as it give a great addition.
I believe that the whole idea behind VOIP is simply to be discribed as additional service but never a solution. so in order to make your service better i advice to add A2billing.
For me it would help a lot as i can give my friends a way to make VOIP calls for prepaid amount or what ever suffecient method.
Are you going to add A2billing? or once have to pay the 45 Euro to get it.
I think as fare as you are on the beta level you have to add it soon befor you shift to higher level so it would be tested.
Any way i would like to hear your answer as it was asked befor but no answer was posted.