AFAIK the standard encryption used by GSM is A5. I've read an article at darkreading.com about hacking A5 with a ~250$ radio device on the fly, so you're not secure with A5. There has to be an app to monitor GSM data (probably the new version of the old field test menu present in old Nokias), but most likely it won't be available for public. However, if you do need to check such things, i'd go for activating R&D mode and looking around what can be achieved, but you'll be on your own on that road.