as there would be no mobile phone signal at cruise altitude,
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.