I know a lot of people don't bother to answer their phone unless it's from a number in their contacts, but not everyone is like that. I almost always answer calls from numbers I don't recognize, and it's often a call that's actually important. A coworker or friend whose number I don't have, the bank calling me about strange activity on my account, a callback about a message I left and forgot about -- who knows? It only takes a few seconds to realize it's a recruiter and say "No thanks, please don't call me again" and I certainly don't want to add their numbers to my contacts list to filter them out. I like being immediately accessible in emergencies, and a minor inconvenience is worth that to me.

Because sometimes they actually have something interesting for me.