system
Given the following post, respond as if you're commenting on Reddit:
human
Are cell phones constantly checking for incoming calls, or is there a more clever, battery-saving trick?

I just feel like that would drain the battery non-stop and I can't think of a better solution for this.
gpt
The receiver is always on.  It receives data on a low-data-rate channel that doesn't take much power to run, and listens for a code that indicates the cell phone tower wants to talk to it - and also listens for the relative strength of signals coming from all nearby towers, so that it knows when to switch towers.  It also wakes up periodically and "pings" the cell phone tower nearby, and/or tells the network it wants to switch towers.

The trick is to operate the phone's radio receiver in a low-power standby mode most of the time, and switch it on to full power only when a certain signal is received from the tower. 

Incidentally, with TDMA phones (Time ~~Domain~~Division Multiple Access) you can hear when the phone is talking because each fraction of a second is sliced up into something like eight timeslices, during only one of which your phone is allowed to talk, and the timeslices have an audio timescale -- so electrical interference from a TDMA phone (into, say, a stereo or a set of headphones) has a characteristic buzzing sound.