Pretty much the first thing that needed to be solved when moving from 1-way pagers to 2-way phones. Pagers could just get a broadcast analog signal and determine themselves if they were the intended recipient. 2-way needed more bandwidth and a dedicated communication channel to a specific device, so broadcast wasn't feasible. Thus, phones would send a registration signal that a tower would pick up, and that specific tower would handle all communication to that phone. If another tower got the registration signal, communication would switch to that tower.

Interestingly enough, there was a period (for a fairly long time) that if you were travelling too fast, you could either a) not be able to register on a network, or b) overwhelm the network with registrations - part of the reason why phones had to be turned off on airplanes