Hi
You can always get paid help from
[email protected], the rates are on the front page of this website.
But in brief
Take the context a2billing-did in extensions_a2billing.conf and copy it down to the bottom of the file and rename it custom-a2billing-did
Then add a custom destination in freepbx, and in the destination, put
custom-a2billing-did,${exten},1 and call it A2Billing-DID
Then test your inbound DID to a FreePBX extension.
If it works, you can then change your inbound route to the custom application - A2Billing-DID which now appears as a dropdown in FreePBX inbound routes.
Joe