If you registered your phone number with the National Do Not Call Registry when the program started in 2002, you need to re-register. Phone numbers automatically drop off the list after five years, in theory to allow people who change their numbers and get recycled numbers to have the option of enjoying unsolicited sales offers by phone :P.
The good news is that it's easy to re-register.Here, have a link.
The good news is that it's easy to re-register.Here, have a link.