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.
So when airborne one's phone may try to communicate with a mast more than 15 miles away and then give up completely even if there is a mast closer.
In my experience flying at 2500' or so results in an intermittent connection. Sometimes one can have a proper conversation with a bluetooth headset but expect to be dropped any time. It is usually possible to get a text out but it may take several tries.
The GSM modem in an iPad gives weather/NOTAM updates via Skydemon .... but again intermittently.