I would think they have you call back to confirm the number is legit.  Once they hear from you, they probably add it to their call list as an actual number and flood you with even more calls.  I get calls from them too and just wait until later in the night when my calls are free to delete the message.

I am receiving calls from this and other phone numbers on my cell phone concerning auto warranties. I have to pay for minutes used on my cell phone and do not appreciate my number being called by solicitors. Every time I call the number back to be taken off the list, I receive several other calls from different numbers, but with the same message. Please help me to get this stopped.

I called this number back after missing a call. It was a recorded message saying that if I wanted to be placed on their DNC list, press 1. No indication of what they wanted or what company it was.