Even if there is no GPS, it should still be possible to tell whether a number is registered/active on the network. When a phone connects to a network (including

Even if there is no GPS, it should still be possible to tell whether a number is registered/active on the network. When a phone connects to a network (including roaming) it's location is stored in the operator's HLR (home location register) where it is used to route calls. I'd assume it stores the time the phone was last seen as well.