From 340 TTT US paperback
"'The God,' Kellhus said. 'He doesn't speak to you?'"
I guess I never thought it through, but Kellhus' 'madness' is characterized by messages from the God and the No-God (if he is to be believed). Thinking back on the conversation, Kellhus vs Aurang/Esmi, Aurang's reaction to Kellhus' assertion that Kellhus hears the No-God is curious. It's at least possible that Aurang isn't faking his terror. Ergo Aurang thinks it is possible for Mog to talk to Kellhus. Ergo, it might actually be possible. I had simply assumed that Kellhus was telling a convenient lie, but he could be getting dreams from Mog all the time for all I know.
Another possibility: Kellhus is aware that his dad tried to manipulate him by sending visions of Mog while Kellhus hung on the circumfix. To throw his dad off, Kellhus acts like Moe unintentionally overexerted Kellhus, so that Kellhus was pushed past the manufactured madness of having visions of the No-God to full on clinical insanity of hearing from God too.