Wandering around at random without paying attention when you're carrying on a phone conversation. Total unawareness of where you're going, what you're doing, or what's going on around you.
(hangs up, looks around) How did I get all the way out here? Must've been on phone autopilot.
Person1: Alex just walked into the room and looked right at me taking cash out of his wallet, I'm so busted!
Person2: Nah, he's just on phone autopilot, he didn't really notice.
You have to stop cleaning while you're on phone autopilot, I just found my car keys in the refrigerator.