USSD (Unstructured Supplementary Service Data) is supported by only GSM Mobile. Unlike SMS it is session Oriented. So If SMS is like an email than USSD is like a Chat. Every USSD message creates a session between mobile and Server. USSD cannot be pushed in very High quantity like SMS, because it creates session and every session has its time limit. Data transfer over USSD is fast in comparison of SMS.
Currently we are providing only Mobile terminated USSD. Although it can be Mobile Originated by Triggering USSD server by any other method like miscall.