High battery drain on mobile devices (phones in particular) when in 'NO-SIGNAL' areas is a common problem with all mobile devices.
Cellular mobile devices have Adaptive Power Control which varies the phone's transmitter power. The software reduces transmitter power automatically in proportion to signal strength from the cellular tower. This greatly decreases battery drain. It's also done to reduce signal interference between mobiles.
When in a 'NO-SIGNAL' area the transmitter goes to full power trying to establish a handshake with the nearest cellular tower.
This is a compelling reason (apart from being told to) to switch your phone to airplane mode when flying.
However, typically the phone sends out handshake signals in bursts, time between bursts increases when no response is detected until the phone gives-up. It then waits until it next detects a cell tower signal. This feature varies between mobile device manufacturers.
