I've encounter several times when I am talking to my boss that his phone rings. He picks up the phone and starts talking. The talking could last for 5 minutes or 15 minutes, depending on the content of the call.
Maybe I should return later instead of waiting next to him endlessly, doing nothing but having frustration? However, I think it is rude to interject by saying: "I should look for you later." Moreover, the matter we were originally discussing is not finished so maybe I shouldn't go?