Probably a debt collector. Had the same problem with calls very early in the a.m. made to my work phone. I asked our IT dept. to investigate, and this is what they told me:
"Someone did a forced call back. This is when someone calls you and then hits the mail button. This forces the person they were trying to call to give them a call back and will not go away until you reach them."
Nasty, if you ask me, and as ineffective as all the other methods they've tried thus far, including the bogus "delivering papers to the clerk" threat.