We know that after the bye, Garrett started relaying the call to Romo. We also know that at times, Jason looked like he was calling plays... There were times when the previous play was barely over before he was holding the playsheet up to his mouth and calling in the play. On those occasions, I don't think there was even time for Callahan to make the full play call and Garrett to repeat it. So I think Jason was calling them himself, at least at times.
We also know Broaddus has strongly suspected (at least) that Garrett was calling plays.
Yet Garrett insisted that Callahan was calling them.
This sounds like a joke, but I seriously wonder if Garrett didn't take over playcalling and they set up the system of Jason-as-relayer so Jason could claim Callahan was calling them and Jerry wouldn't know any better.
I mean, that's so wheels off, but it would explain why it looked so much like Jason was doing it.