Hi,
Call Starts as an Isup Message and it lands up in GMSC of A-party.
Now GMSC of A can be configured to fire SRI(if possible) to the B partys HLR which returns back with the MSRN. GMSC of A can validate if this MSRN belongs to a local range and decide if the call can be routed locally.
This approach depends on whether B partys HLR recognises a SRI query coming from a foreign GMSC.
Rgds