What laws apply to telemarketing?

Summary:

The Telephone Consumer Protection Act and Telemarketing Sales Rule may both apply here.

From www.business.gov:

Telemarketing is a great way to directly reach existing and potential customers, but it is also subject to additional rules and regulations due to a history of overly aggressive selling tactics and deceptive practices. Commercial telemarketers are regulated by federal and state laws that establish curfews, do-not-call lists, and other requirements aimed at protecting consumer privacy.
Telemarketing is regulated at the federal level by two statutes: The Telephone Consumer Protection Act of 1991 (TCPA) and the Telemarketing Sales Rule (TSR).

The Federal Communications Commission (FCC) weighs discusses its telemarketing policy here: http://www.fcc.gov/cgb/policy/telemarketing.html

A great resource for further information is provided by Business.gov, although some exceptions may apply for nonprofit organizations: http://www.business.gov/business-law/advertising-law/telemarketing/