I imagine that the signal would reach upwards a considerable way as it is unimpeded by obstacles, although some operators apparently shield upwards in order to allow a stronger signal to go out horizontally - I was told this by someone whose company put up some of the mobile networks in Central Africa.
A 'friend of a friend' says he was on a flight between CPT and JNB crossing the Karoo in cruise, probably at FL 37 and he swears his neighbour's 'phone rang and the guy answered the call. Apocryphal? I don't know.
I was on a flight into NCL the other day and just after we started our descent over Sheffield, a phone rang, I'd guess we were at FL 20-25 at that point? Someone will have a better idea of that than I do.