Initiate() does not return until a USSD notification (event) arrives. Firing the callback moves the state machine of the USSD core so it expects one ON_USSD event to arrive.

In some networks you will receive the ON_USSD event before the reply to SEND_USSD, so if you wait for the SEND_USSD reply to make the callback, the USSD core state machine will not be in the right state and it will not think that the incoming event is related to the previous USSD request, so it will generate a NotificationReceived/RequestReceived signal and Initiate() will still be blocked.

The best way to thing about this is the the ON_UNSOL event is effectively the network reply to the SEND_USSD request.