I don't like it when people intentionally don't answer their ringing phone. Especially if it's because they don't recognize the phone number. You never know when somebody is having a problem/accident and may be using someone else's phone to call you. If you answer it and they just want to bullshit, then you can hang up on them.
It's different if you don't hear your phone ring or if you're in an important meeting or doing something where you cannot talk. Under normal circumstances, just answer.