It has nothing to do with waiting too long. If you are on one call, get another call, that option should always display (assuming your display is illuminated). Sometimes I've hung up with one party, but the phone doesn't know that and it stays there until I end both calls. Like I said earlier, WidgetLocker screwed mine up, but if you don't have that, then you may want to reboot or, as a last resort, pull the battery.