I am currently a Verizon customer, but I'm thinking about switching to TMobile. I've done their testdrive, and it has worked well for me. However, in case things go wrong, I will not be under contract with TMobile, so I'd like to option to go back to Verizon. My understanding is that CDMA phones are effectively a superset of the GSM phones. i.e. CDMA phones work on GSM (presumably since GSM is more common worldwide and this allows better roaming), but GSM phones cannot work over CDMA. Is that correct? Can a CDMA phone do everything a GSM phone can but not vice-versa? If so, I guess I'm better off buying a Verizon phone before switching, then terminating my contract and getting a TMobile SIM for my phone.