Poor or non-existent signal strength will cause the phone to
consume extra power.  When the phone is idle, the phone may
spend 15 minutes or so constantly searching for a usable signal
before automatically dropping back to more infrequent searching
to conserve power.

During calls, poor signal strength will cause the phone to
transmit at a higher power output.

It's also common for the "gas gauge" which displays remaining
battery capacity to become miscalibrated, grossly
underestimating remaining capacity.  To recalibrate, simply let
the battery flatten until the phone switches off of its own
accord.  Then give it a full, uninterrupted charge.

John