Just side note - modern (aka produced in last 10 years ;P ) cell phones *do* receive special signals from cell towers even when turned off, after logout signal. They (turned off phones) normally *don't* send any signals - but, filtering conspiracy theories for some meritocratic content, there *is* possibility to include "act-dead wakeup", upon getting special, secret kind of signal from cell tower.

Still, AFAK most phones stop actively receiving any signals when battery is pulled out. Still, in modern technology era, and considering how low-level and low-bandwidth those signals can be, i would not be surprised, if some chip on phone would contain special backup power *just* for receiving this kind of signals.

Ho ever, I think it's not possible yet, to create such small and "sneaky" power source, than can power-up phone systems so much to *send* answer for that signal. Of course we're still talking about battery pulled-out scenario.