19 CFR 111.3: Customs business.

(a) Location. Customs business must be conducted within the customs territory of the United States as defined in § 101.1 of this chapter.

(b) Point of contact. A licensed customs broker, or partnership, association, or corporation, conducting customs business under a national permit must designate a knowledgeable point of contact to be available to CBP during and outside of normal operating hours to respond to customs business issues. The licensed customs broker, or partnership, association, or corporation, must maintain accurate and current point of contact information in a CBP-authorized electronic data interchange (EDI) system. If a CBP-authorized EDI system is not available, then the information must be provided in writing to the processing Center.