Strange, that's not in line with what a phone should do.

Every second (or even twice a second), a network connected phone in idle wakes up the receiver, listens to find out if there is an incoming call, and if not, it goes back to sleep. This is around 2 to 3 of the idle current of 6 or more mA that you see in the battery app.

If there's no network, the phone will still wake up regularly (not sure if it's once a second, but not likely to be, and not likely to be more) to see if there's a base station it can connect with. If not, it should go back to sleep until the next one.


What I can imagine, is that your phone is actually getting a signal, and sending out a signal to connect, (sending is expensive in terms of battery/current), but not getting through, etcetc.
If your phone shows reception close to the window (helps being on the correct side of the building), try having it connect (stay there for a few moments), then see if it keeps the connection if you move away from the optimum spot.

It may have trouble to send, but (depending on the network), that may take hours before that happens if you don't move around in a big way.
Now, it does mean you'll probably not be able to receive a call properly (in that location), but it also means you won't have to switch to flight-mode and subsequently forget to turn it back on, missing all calls.