On the other hand, now that I think about it some more...

If you're in fringe areas where there's not very good cell coverage, a phone might overwork its battery trying to maintain a connection. The various signaling methods used by the phone will change dynamically depending on which cell towers it can see and what their signal strengths are. Some of those signaling methods use more juice than others, especially if the phone has to boost its power output to be "heard" by a given tower.

What's your signal strength indicator usually look like?