It seems as though the following should work:
is whatever specific number you want to use. The **2 that comes before your desired number is optional, in the event that you want to route the callback through SP2.
The good news is that I tested this using an anonymous call block, and the AA still performed the callback to the desired, target number. The only problem is that I am unfamiliar with the AA and also do not have sufficient landlines available right now with which to do more testing. For reference, I read this from the OBi Device Admin Guide's AA Callback Service
A rule can be added to the InboundCallRoute parameter to have the AA call back the caller’s or any other number, if the caller hangs up before the AA answers. The rule should indicate that “aa(callback-number)” is the target destination of the call, where callback-number is the number that the AA should call back if the caller hangs up before the AA answers the call.
Hope it helps,