=How to say "No" politely in English=
A: Hi Taro, I suddenly need to go see a client. But I was supposed to call Mr. Smith. Could I ask you to call him for me?
B: Oh sorry, but I'm not sure that I can. Because I'm also heading out to visit a client soon.
A: Oh okay, never mind then.
B: Well, hold on. Maybe Mr.Suzuki can do it, I heard that he'll be in the office all day today.
A: Great, thanks for letting me know. I'll ask him.