Some people have no boundaries. I used to work at a restauarant, and it got to the point where I never, ever picked up the phone on my day off. "other plans" was not considered acceptable if they actually got you on the line. The solution was, don't pick up the phone. This was before voice mail, called ID and all that.
Nowadays, there's no reason on the planet to answer a call you don't want to take. We don't keep a phone in the bedroom for starters, and I never ever pick it up in the middle of the night. Nowadays with kids in college I get up and check the ID if it rings in the middle of the night, but I don't answer it.
There's voice mail, caller ID, turning the thing off, etc - lots of options besides answering.