Nigel on Draft...that's why I asked for a 777 guy to answer. I know first hand about 737/747/330/340 etc.
On at least two of those you cannot access the c/b in flight, NOR CAN YOU SELECT VOX ON COM 3
On the 737 and 747 switching 3 to vox does not lock it there necessarily...some radio options default back to data after a few minutes.
See now why I wanted a definitive 777 answer?