The handset for a Land Rover Range Rover P38 transmits a coded message to the vehicle's computer and the code changes every time a button is pressed on the handset. If the handset is operated too many times out of range of the vehicle, or if the handset batteries are removed for longer than a minute, then you might need to re-synchonise the handset to the vehicle. You also might need to perform this operation if you purchase a new handset.
To synchronise the handset:
1. If the Range Rover is locked, press the "unlock" button on the handset and then unlock the vehicle using the key on the handset.
2. If the Range Rover is unlocked, press the "lock" button on the handset and then lock the vehicle using the key on the handset.
That is according to the owner's manual. The way I learned to synch the handset is different: stick the key of the handset into the driver's door lock, and turn to the right, hold for 3 seconds, then turn all the way to the left, hold for three seconds, and finally bring it back to center.
You cannot synchronise the handset if the batteries in it need replacing, or if the Range Rover's alarm is armed.