When faced with that situation I am usually straightforward and blunt. It happens to me most frequently when somebody rings during dinner. I say, "I'm in the middle of tea just now. I'll call you later on." Then they just carry on flat out as if I hadn't said anything. It's really very annoying. However, I have found that such people usually have very thick skins and saying, "Look, sorry, busy just now." - Pfffft - and hanging up in no way causes any offence. It is commendable that you consider the feelings of other people, but with these garrulous types it isn't necessary because they do not register any rudeness - they really don't, you know.
Look at it this way, how often to you harbour a smouldering grudge because somebody gave you the bum's rush last month ? Chances are you'd just chalk it up to their being a bit irritable and forget about it. Your time is as valuable as that of the next person; be assertive.