Another interesting point is that when travelling the phone is always on the lookout for the neighbouring cells so as to switch over when reception is better in the new cell.
The phone receives this information from the cell transmitters (passive affair) but when switching over cells it transmits a burst of data (active battery bashing activity). The cells are small in towns and built up areas, larger in country areas. So, your phone does a lot more work when on the move; in built up areas it may be swapping cells ten times or more per hour. This is why the traveller finds that the battery life is less, even though they make and receive a similar number of calls as when static.
Another tip is always try to keep your mobile phone clear of obstructions, as its transmission activity is geared to its perceived signal strength. The little data bursts are only tiny milliwatt affairs when it sees a good signal from the cell transmitter. If it's in your pocket or a cupboard or otherwise hidden form the cell transmitter its data bursts are really strong to try to get through. This uses up more battery power.