On Virgin flights you can use wifi on your devices, think voice as well. In the air, the phone wouldn't get a normal mobile signal anyway, but the flight has its own "network" which you can connect to and use. Think its the same cost as international roaming charges.
I rang my colleague the other day whilst waiting for him to turn up and it was ringing...then I saw him walking towards me on his phone to someone else, but it hadn't gone into voicemail. Doesn't the phone network record the "last" state of the phone? So if the last state was connected and not on a call, then it suddenly loses contact without being able to send an update (like I think it does when switching off etc), won't the network think its still on? Presumably though it would time out eventually.