If you answer the phone, they know someone will answer the call and will sell your number to others.
Legally a telemarketer CANNOT contact you if you explain you are on the DNC or do not wish to be contacted again.
If they continue to contact you beyond that point, you can ask for their information and threaten to contact various regulatory organizations.
You can also try contacting AT&T and see if they can pull some weight in stopping the calls.
Basically the law is 100% on your side, and if you are getting repeat telemarketer calls, or telemarketers who are selling your name to other businesses, you have every right to demand they stop.