I just equate watching the phone ring and not answering it with if one of your friends walked up to you in the street calling your name repeatedly, but you just turned the other way and pretended you didn't hear them. I agree that if I'm busy or in a bad mood then I won't answer the phone, but if I'm not doing very much and my friend starts calling me, I'll answer.
Usually if my friends want to say something general to me, they'll message me on Facebook, and I'll usually reply within a few hours. So if they call me, it's often because they want to talk to me about something more instant - whether that's an emergency, or whether that's asking if I want to meet up with them in three hours. So to me it generally makes sense to answer.