I suspect the problem is not so much the lack of transmitters as the fact that from the top of the hills, the phone can 'see' too many transmitters (and v v). The system tries to use the transmitter giving the strongest signal but this will vary very rapidly if too many transmitters are in range. The system gets confused and instead of being passed smoothly from one transmitter to the next, the connection is lost. Well that's my theory anyway - from someone with a little knowledge of how it all works (which as we know is a dangerous thing!).