It's totally a software thing. There are things called Number Assignment Modules. The NAM is the electronic memory in the cellular phone that stores the telephone number and an electronic serial number. Phones with dual- or multi-NAM features offer users the option of registering the phone with a local number in more than one market.

I have this set up so I can switch to NAM1 for Verizon when in the States and NAM2 for Smart when in Belize. Any cell phone shop here seems to be able to do the programming, so it doesn't seem like CDMA phones are locked.