I'm sure the actual comms capabilities of AF1 are strictly classified, but I'm willing to bet they can get on any frequency they want, at any time.
While You-know-who was in Maine, there were Combat Air Patrols flying about - there was a little traffic on this on the Scan Mass.boards. It seems the NJ ANG, Shaw AFB and Otis a/c all got into the act. Something to keep in mind the next time he's around. Most times its pretty routine, unless someone is foolish enough to wander into the restricted zone.
73s Mike