Hi
Please be aware that there are a lot of people out there who will offer you help and consultancy, but please do your research first. Check the number of posts they have made on these forums as a start, and see the answers they have provided. If their first post says "How do I install A2Billing", and the second post says "We offer consultancy on A2Billing" - Walk away in the opposite direction very quickly, as they probably do not know what they are talking about. I would say that to employ an A2Billing consultant from these forums, you need to be looking at someone with at least 100 posts to their name, with some good answers.
See
memberlist.php?mode=&sk=d&sd=dThis not only tells you how good they are, but also they are likely to want to make sure everything is as it should be as their reputation here will be important to them.
Note that Star2Billing has been set up by A2Billing to provide consultancy and provide a revenue stream to support the future development of A2Billing.
In respect of your minutes to a SIP provider, there is no way to limit them, you should go for a proper wholesaler.
The customers can have a package offer attached to them, so once they have used their free minutes, they are charged for each subsequent call.
Joe