K.,
I think this is common, and he may outgrow this, but there are lots of things it seems to me, that are going on.
He has a new baby sister, adults are talking about a new house, and he could be one of the types of personalities that he has to know what he is doing, when he is doing it, a schedule I guess. My daughter was like this, still is. She has her entire week mapped out, and if someone throws a kink in it, oh goodness, look out, and she's 23 for crying out loud. Never was one to fly by the seat of her pants, but I created that monster in her, but she's my monster :) LOL. With three children, I had to have some sort of order to our daily routine.
Anyway, I would continue with the making him say "excuse me please" But I wonder, have you also considered talking to him, about what you are doing, and why you have to talk to people? Like realtors, etc on the phone. Does he know that you are planning a move? And have you and your husband discussed it with him. Kids are so smart. But this can be an anxious time for him too. And I think they feed off our stress as well. I know mine did. I know there are books out there on moving, etc, that will help explain things to him. And maybe if Dad has a special time with him, just for being a great kid.
Maybe if he had a special something to play with only while you were on important calls? Then if he constantly interrupted, he was sent to time out. Also, I wouldn't put off punishment (not saying you are) until you are off the call. If the behavior is happening now, the consequence needs to happen right away. IMO.
Kudos for trying and understanding him. I know this is an extremely special time for all of you.