Let me check I understand what you mean by ANI callback first. You want each customer to have a dedicated DID which, when called, always rings back to the same predetermined number?
If so I believe currently the only way to do that with a little bit of help from asterisk dialplan. Force calls to a certain number to enter A2B with a predetermined CLI and add that CLI to the customer's account.
Code:
exten => 441234567890,1,Set(CALLERID(num)=4412323232323)
exten => 441234567890,n,Goto(a2billing-cid-callback,${EXTEN},1)
Now calling 44123457890 from any CLI will trigger a callback to 4412323232323.