Hi
This is not really an Asterisk forum, so in terms of how you take a DID, and deliver it an asterisk server, is outside the scope of this forum. However this link may help.
http://downloads.oreilly.com/books/9780596510480.pdf
Once the call is in A2Billing, as previously described, create a DID group add the DID ,then in the add destination under DID, just point it to the calling card account number and tell it what technology to use, e.g. sip/12345678 for calling cards, or sip/
[email protected] for sip devices where 1.1.1.1 can be the IP or the hostname.
Joe