Agree with Tumtiddle.
From my knowledge of cellular comms, the phone is constantly in rf contact with towers, both sending and receiving data when the phone is locked, because there are two channels that the phone communicates on - one where you speak to your end party, and one where the 'background' comms work to connect calls, ensure tower linking, handshaking etc.....
Only when the phone is in Flight Mode, or Off, is when all communications are disabled i.e. no wifi, data, voice etc....