I run an ISP that is starting to provide VoIP services to our customers. We have most of the technical aspects figured out but are stalling a bit on billing. Is there a good billing package for Asterisk that can bill a pre-paid account on INBOUND calls. ie… All calls recieved by xxx-xxx-xxxx are billed against a pre paid account associated with that phone number?
Flatplanetphone.com has a great Virtual PBX that you can work off or lease their software.
(Yes I work for em).
Having tried most of the billing solutions out there, there is in my opinion only one open source implentation that works as expected , and that is A2B (mentioned in a previous post). As a guide I always look at the support forums to see how well it is supported. The A2B forum gets 10+ posts a day and questions actually get answered!!! (unlike Openser 
)
We handle some 10k+ calls/day for abount 8 months now and it works a treat with low cpu overhead
http://forum.asterisk2billing.org/
Regards