(a) The rules in this part shall not apply to customized equipment or services that are not offered directly to the public, or to such classes of users as to be effectively available directly to the public, regardless of the facilities used.

(b) A provider of advanced communications services or manufacturer of equipment used for advanced communications services may claim the exemption in paragraph (a) of this section as a defense in an enforcement proceeding pursuant to subpart D of this part, but is not otherwise required to seek such an affirmative determination from the Commission.