There's no greater first-world anxiety (save for the incoming call music on Skype) that causes greater distress than the unknown caller. While it's typically a telemarketer or simply a friend's new number, the mind always jumps to the worst possible scenarios, which leads to a very distressing pick-up.
Apple, bless its corporate soul, is eliminating this anxiety by introducing a feature in the upcoming iOS 9 update that will suggest who the unknown caller might be. This works by scanning your email and checking to see if the unknown number has ever contacted you before. Business Insider reports that "Apple says it does this all anonymously, so your data and personal information is never compromised or given to a third party."
So, you can check that first-world fear off the list.