An Impossibility

The records cannot be falsified. They are indisputable.

The records on the only way into the room are secure. They show that he was the only one in the room.

The records of what was said in the room are unusual. They appear to have two voices speaking in turn, discussing an impossibility.

The records relating to radio and associated wavelengths inside the room are clear. They prove there was no outside way to get a voice in the room.

Ergo, we conclude that he was speaking in two voices. The alternative, that the records are altered, is clearly impossible.