If that's the case, I don't understand why he didn't just dispose of the phone with her body. Why have KK come to Colorado just to take the phone back to Idaho and text KB's employer/PF she won't be back for a week? That elaboration isn't necessary.
Wouldn't it have been easier to just attempt to drop the baby off after a few days, call her mom and ask her if she has heard from her, and report her missing himself?
To be frank, it is the whole involving KK and getting the phone to Idaho is what really broke this case wide open. Her body still hasn't been found and if he is the sole killer, his elaboration was his downfall. In whatever manner her body left the house, it was done discreetly. In whatever manner she was disposed, it was done discreetly. He wouldn't be in jail right now without the elaboration. Right now, she'd still be a missing person like the many others who hold such title.