Ooooh - lot's of ways!
If you are in the office: "Oh, I'm so sorry, my boss has just appeared and needs to talk to me urgently. I'll call you back when I can. Goodbye."
If you are at home: "My goodness, there's someone at the door. I must go."
"Sorry, xxxx, I must go - there is something burning in the kitchen"
If you are on a mobile phone in the street, switch it off - he is not to know if you are in a black spot (no radio rception) or not.