Go read this interview with one of my favorite social critics: Miss Manners. A quote:

TAE: Speaking of technological advances, is "call waiting" rude?

MARTIN: Absolutely. It presumes a policy of last come, first served, and that is against all the rules of polite precedence.

TAE: But what if there's an emergency?

MARTIN: Telephone companies will still break into a conversation in case of fire, crime, or medical emergency.

TAE: So you'd rather get a busy signal?

MARTIN: A busy signal is a quick, efficient, and free way of notifying people that one's line is busy. Why anyone would pay extra for the privilege of offending current callers while having to make this explanation himself, I can't imagine.