With GSM there is a problem in that the protocol assumes the signal takes less than a certain time to go between mast and phone. If the signal takes too long it does not work. I seem to remember the maximum distance is ~15NM.
My recollection is 35km (I've got a large pile of the ETSI/3GPP standards on my desk, the details are in there somewhere, but I'm afraid I'm not going to go looking right now).
If the phone is a long way from the mast it has to transmit its signal early so that the mast receives signals from all phones properly slotted together, and there's a limited number of bits in the protocol for telling the phone
how early to send its signal. Plus, before you get that far, there's a limit to the amount of slack in the timing for the initial contact from the phone (before it's been told how early to transmit).