You may call anywhere in the U.S. as excon correctly noted. Business phones cannot be placed on a "Do Not Call List" as residential and cellular phones may be. You must not call any number listed on such a list.
The national list is administered by the Federal Communications Commission. You can learn more about it at
www.DoNotCall.gov Some states also have "do not call" lists and laws.
Additionally, while you may call anywhere not listed, some jurisdictions to have restrictions: Wisconsin requires the posting of a bond before you may conduct telemarking campaigns in that state. (Personally, I think their law is unconstitutional and a violation of the Interstate Commerce Clause, but no one has challenged it yet (as far as I know); it was only enacted last year).
Some other states place restrictions or limitations on the hours or days you may call, and still others restrict or regulate the use of predictive dialers (auto dialers) and/or recorded messages. The attorney-general's office in each state can provide you with information on their respective rules.
Note my profile shows I am a marketing executive. More precisely, I am national sales director at a large technology supplies wholesaler. The vast majority of our business is b2b, and done over the phone.