You could match the CLI in the Asterisk dialplan and based on this invoke A2Billing with different destinations. In the following example 011111111 is a CLI and 0121212 is the destination it calls.
Code:
exten => _X./0111111111,1,Goto(a2billing-standard,0121212,1)
exten => _X./0222222222,1,Goto(a2billing-standard,0132323,1)
[a2billing-standard]
exten => _X.,1,DeadAGI(a2billing.php|1)
; (or however you usually launch A2Billing)
There is related functionality already in A2Billing, but I don't think it can easily be used to do what you want here. See Ratecard -> Create new Ratecard -> CallerID Prefix.