Hello Vijay,
The calledParty address of SCCP should include IMSI, refering to HLR which it belongs, with numbering plan set to E.211.
The USSD Phase-1 doesn’t support sending msisdn as part of USSD messages, where as USSD Phase-2 supports the same.
In case of USSD-Phase 2 we can use msisdn ( under E.164) addressing format to reach to respective HLR.
-Thanks
Hari