Caller immediately hung up when I answered. I called the number from my cell phone, and it immediately went to a voice mail message that said, "Thank you for calling back."  If you are not interested in our promotions and wish to be put on our 'do-not-call list', please press 1 now..."  

I never oblige these message requests with a response.  I want them to think that I dialed a wrong number, thus not confirming their initial call to me.  That would give them both my cell# and my landline# that they originally called.