Got a missed call on my cell phone from these people.  No message left and I've been getting a number calls from unfamiliar numbers recently.  

I don't pick up from callers I don't know and these people never leave a message.  I finally decided to call the number that was picked up by my caller ID and got a message to the effect of "I'm sorry, but the voicemail box you called has not been set up yet.  Goodbye."  Then it hangs up.

What the heck??