When the President is aboard, the callsign becomes "Air Force One." I'm confused why they'd be using the "SAM" callsign.
If the aircraft is moving once POTUS has deplaned, then it's "Special Air Mission XXXXX." An example of this was when Nixon resigned. He flew on the old B-707 (retired after Reagan's presidency) back to California. It was identified as "Air Force One" until the time that Gerald Ford was sworn in. At that moment, it became "Special Air Mission 27000," as the sitting president was no longer considered onboard; just a former one.
Fox News Channel carried the arrival of VC-25A (registration 28000) live, which is how I'd noticed the number on the tail.
"In this present crisis, government is not the solution to our problem - government IS the problem." - Ronald Reagan
Comments made here are my own and are not intended to represent the official position of Alaska Air Group