1-888-382-1222. It is the National "DO NOT CALL list.
It will only take a minute of your time. It blocks your number for five (5) years.
HELP OTHERS BY PASSING THIS ON TO ALL YOUR FRIENDS OR GO TO:
EDIT: This may be an urban legend (see about.com: Cell-Phone Directory?), but it's still a good idea to add your cellphone to the don't call list. Probably also a good idea to phone from your work line as well (we get a lot of telemarketers at the record store...it's a pain).