Same scenario for me.  I did not answer.  I called back and got the Magic Jack message but the mailbox was not full.

Did recognize the number, so after they called, I tried to call them back on my work land line. It just rings and rings and then says "The Magic Jack customer is not available and this user cannot accept any more messages in there mailbox."
I have no idea who this is.

did not recognize number so did not answer; no caller ID or message