You probably won't get a phone connection at cruising altitude (at least in the South of England). The reason is, oddly, the reverse of that stated above. It's not that the signal is too weak - it's that you can see too many base stations.
On a network the base stations re-use frequencies - but arrange it so that a phone can't see 2 base stations using the same frequency from any one point FROM THE GROUND.
In the air your phone can "see" much further (line of sight). If the phone can see/be seen by 2 base stations the network interprets that as 2 phones with the same serial - which must mean it's been cloned. The network then reacts by preventing "both" accessing the network.
The only way of preventing this is to fly "really low".
OC619