GSM: 1) You can transfer your number to any cellphone by simply popping in a SIM card, assuming the card fits. 2) You can do voice and data at the same time. 3) It's the most widely used technology among all the carriers in the world. CDMA: 1) You have to get your carrier to add some ID belonging to your cellphone to their database before you can use the phone on their network, unless the phone's already configured to run on that network. 2) You CAN'T do voice and data at the same time without two radios or whatever the iPhone 5 has. 3) Unless your phone can be used over a GSM network, the only other country I can think of that uses CDMA is China.