Ever watched any American movie? The girl usually writes the number on a back of a receipt or napkin, gives it to you and immediately leaves.

:).

I do generally agree with the OP though. I need to have a way for unknown but legitimate calls to reach me. To use actual examples that happened to me - sometimes there's a problem with my on-line order, or with some form I submitted to the government, and some clerk from a company/govt office calls me to set things straight. I don't have, and couldn't possibly have, an idea from what number they'll call, but getting that call is a difference between fixing a matter on the spot, vs. wasting me (and them) half a day + incurring some extra wait time the next week.