I truly believe that the phone calls they make between boarding and taxiing are a power trip to let the other passengers know that they are important businessmen. I have never once overheard a pre-takeoff call that was urgent. It's for show, and it's always the same: "Hey, it's me. I just got on the plane in ____. Should be landing in ____ and I'll call you then. Oh, did so-and-so get back to you about ____? I'll have to deal with that later."