Voice in your head and convincing followers
After a prophet hears a voice of prophecy what can he do with it? First he has to find a way to convince others that he really received something.
How would a prophet know where the voice in his head comes from? As far as he is concerned it started in his brain. Since he just receives messages, he cannot know where they get send from. A communication technology that resembles voices in your head might not be very convincing.
The message itself could reveal an origin. The sender might include information about who he is, the intention of the message, what he wants the prophet to do etc. It is up to the receiver to believe and accept it. If the information however is conflicting with the prophet's beliefs, outlook on life, etc, he might reject the message.
Even if there would be a reliable way to broadcast a message directly into a person's brain, it might still be hard to proof to others that it really arrived. Due to the uncertainty surrounding this kind of communication there might be no real use for sending messages this way. It seems to be far too unreliable.
Only if there would be no better alternative, this technique, if it really gets invented, might be considered.
From the receiving point of view, a prophet and his followers must accept the authenticity of the message before they will contemplate doing something with it. If nobody is convinced that the prophet speaks the truth, the message will be rejected.