I have found that a lot of people are using a new cell phone pattern:
Keep someone you don’t like in your phone contacts
This came up when I was with friends, and one of our phones rang. My friend answered, had a conversation which he really wanted to end (it was obvious), and then afterwards quickly put that persons phone number into his contact list.
Another friend wondered why he would put his arch nemesis into his cell phone.
I knew immediately. From now on, this person is going to show up on caller ID, and my friend can safely ignore it!
As I have talked to more people, they often follow this rule. Some variations even include actively getting the persons number from their or via a between friend, allowing you to never even get that first call!