Ask for permission to talk.
The first thing out of your mouth once you introduce yourself and briefly explain what you want to talk about, is ask, “Do you have time to talk right now?”
The recipient will be pleased that you asked, even if s/he really can’t talk right now, probably there’s something else going on anyway. (Isn’t there always something going on?)
So, you set up another time to call. But you ask the next time too, just to make sure.
Your mother taught you good manners for a reason, so you could lot’s of friends and colleagues.