To route all SP2 calls to the AA, simply set SP2 Service -> X_InboundCallRoute to {aa}
However, the path you are using (needing three-stage dialing) may be more complex than needed.
If you are in the US, just call your GV number and at the main voicemail menu, press 2 to make an outgoing call. This does not involve the OBi at all and should be more reliable with better quality.
Otherwise, if there are Localphone and/or Rebtel gateways in your country, you can set it up so that calling a local number will directly ring your Callcentric iNum and route right to your OBi's AA. Calls to iNum are free with either.
If neither of the above will work for you, please post details.