Its up to the carrier. CDMA carriers used to be really strict about not activating phones that they didn't sell new. You can flash a phone to another carrier's software, but its still up to that carrier whether or not they will activate it. Before buying any CDMA phone I highly recommend calling your carrier's customer service number, and give them the ESN# of the phone you are looking at, and ask them if they will activate it.

The Nokia GSM phone can still be used on GSM networks, but sadly, the CDMA models cannot. CDMA carriers use the device to locate itself with internal GPS, and transmit its location to the call center in the event of 911 calls. Older CDMA phones are not E911 compliant. This is not needed on GSM networks as the device does not need to know where it is. In the event of a 911 call the user's location is triangulated by the network and towers.