mystery caller

For over a week, I've been getting mysterious calls on my cell phone. They are always from slightly different numbers, with area codes I don't recognize, and whomever is calling never leaves a message. I don't usually answer my phone if I don't recognize the number, preferring to wait and check the message. I thought for awhile it might be the student loan people because my loans were a week late. But I paid my bill and the calls kept coming, and anyway, the student loan people are never shy about leaving messages. Finally today I answered. Why not? Let's solve the mystery. And do you know who it was? Nobody. There was silence. So I waited. And waited. And waited. Nothing. WTF? Then, a few minutes ago, while I was in the bathroom, the phone rang. THEM AGAIN!?!? WHO IS IT?? WHAT DO THEY WANT FROM ME?? I may never know.