Do you expect that they wouldn't be either calling from their own phone, calling more than once, or leaving a voice mail? There are two people who I would lay money on knowing my phone number by heart, and only one of them lives within an hour of me. Everyone else will be stuck looking it up on their phone.

I often go out without my phone, have the ringer off, have a dead battery, or ignore the ringing because I'm driving. It's ridiculous to assume that I'm instantly available 100% of the time, or that I wouldn't treat different incoming numbers with different priorities.