I *do* have an answer for you, and not anecdotally. :-)
When a cellphone or pager is locked up with its system, it goes into a low power idle mode where it monitors one timeslot (generally, out of 6, I think) on the system's control channel, looking for a packet that says *it* should wake up and ring. The other 5/6s of the time, the radios are all shut down and don't draw power.
When it can be in this low-power mode, the battery life is what you'd expect.
When it can't find a system to lock up to, then it has to listen to *all* the timeslots in idle mode, not just one, and the power consumption at idle is something like 6 to 7 times as high.
This apples to modern CDMA and GSM air interfaces, as well as the ones used for paging in the past, POCSAG and FLEX, and probably others.
I had a PrimeCo/Verizon CDMA phone (the QCP-1900, by name) surprise me this way on a roadtrip about 10 years ago; it was the first time I took it out of the Tampa metro (which was all they covered at the time), up to Ocala FL. Was dead by the end of the day, when 2-3 days was the norm.