My feeling is that receptionist is just sick and tired of hearing the same old stupid question.  You really want to scream "how the heck should I know how long he is going to be on the phone" but you respond politely instead.  I mean, really, no one can predict how long anyone is going to be on the phone.

There is one stupid question in every industry that you have to constantly reply with a polite answer.  The question I always used to get when I worked at the bank was, "What do you think the exchange rate will be tomorrow?" when ever someone came in to exchange Canadian cash for US.  I really wanted to say something like "if I could predict exchange rated do you really think I would be working as a lowly teller?" but I would always jokingly respond "well, my crystal ball is broken".