Unless I'm paid to be available, I'm not taking the call.

Businesses buys your time according to the contracted deal; a set amount of time, to do a certain task(s), in exchange for money. Now, no business I know of gives out free money, even if you ask. So, why on earth would any one give them extra time on a task for no money?

Try it. Go ask your boss for some money for doing nothing. In fact, phone late at night and ask...