While I was eating dinner, the phone rang. I thought it might be a friend calling for help on a computer problem. No, it was a recorded telemarketing call. I hung up.
As I sat back down again, I thought if a person had been on the other end, I could have responded, "I was expecting an important call." It would have been a true statement, even if I had not been waiting for a call.
As I get up, I assumed it was a friend calling. If it had been a friend calling, his or her message would most likely have been far more important than the telemarketing call, even if it was just to say hello.
So, the full "truthful" and polite response could be:
"Oh, I'm sorry. I was expecting an important call. I must hang up now." Click.