Sorry, I'm tied up at the moment. May I call you later when I'm done?
Look, I'm all tied up at the moment. Can I call you back?
VERY frequently used! At home and at the office.
Note: "Sorry, I'm tied up at the moment. I may call you later when I'm done - noblesse oblige."
This makes it sound as if you are a very important person, and may...I say, may ...deign to return their call!
noblesse oblige :the inferred responsibility of privileged people to act with generosity and nobility toward those less privileged
deign : do something that one considers to be beneath one's dignity; to condescend to do.