USSD Traffic analysis

Hallo all. I have been playing around with OsmoscomBB for GSM voice traffic and capture. My interested has now stemmed into USSD traffic. Whenever I initiate a ussd session, I get the error "Session Terminated". Has anyone attempted to capture ussd sessions using the Motorola C123 or do I need specialized equipment for this? Also, am looking into researching more into ussd because here in Africa, specifically Kenya, there is a proliferation of financial services over USSD and this begs the question just how secure is it? If anyone on the list might have done a bit of digging around I'd really love to share learnings and insights. -ty