there are some companies that scan for phone lists if the phone number is valid, did some on answer does it have vioce mail they they sell the complied list to telemarketers.
Called my cell phone to sell car warranty. I picked the option to talk to a rep since they had called me in the past and ignored my request to remove my number from their call list. I asked the rep the name of their company and she said: "warranty help desk", then she hung up on me as soon as I asked for their website!