As an American who has had cell phones since the 90s, I have a little different take, as here in the States there are so many telemarketers that I frequently choose to let unknown callers go to voicemail. If they don't leave me a message, then clearly it wasn't important enough for me to worry about.

I made the decision a long time ago that my cell phone is for my convenience. If someone complains that they can't reach me, they can always leave a message, and I'll get back to them. If they choose not to, that's on them.

I refuse to be a slave to my phone. I have a life. ;-)

As for being polite to callers, that applies to life in general. We get back what we put out there, so treating others politely is in our own best interests, and it helps to make the world a kinder place.