I keep getting calls from this number and another one on my cell.  When I try to call back it tells me that "the extension is not available" and then disconnects me, or it says "hang up and try your call again."

Who the heck is this?!?!?!

Calls me, I answer, I do not get a response. Then the number hangs up on me.
I call back, a recording said:"Destination number is not assigned."
Then stop calling me!!

I answer, I do not get a response. Then the number hangs up on me.
I call back, a recording said:"Destination number is not assigned
what they freaking want??