Not sure this is true. In big cities there are basestations peppered all over the place but in rural areas they are often far apart, so they need to have a long range. According to The Wikipedia "The longest distance the GSM specification supports in practical use is 35 kilometres (22 mi)." Vertical distance will be less because of the way the antennas are designed, but it could still be close to cruise altitude.
.......and from the same Wikipedia, (
http://en.wikipedia.org/wiki/Mobile_phones_on_aircraft)
The maximum speed of travel in a mobile phone system is limited by several factors: frequency changes, rate of change of timing offset, etc. The speed of an airplane often exceeds these (typically phones are designed for use in a fast car) which means the phone will fail to register to the network and retry registration repeatedly