47 C.F.R. § 64.1506
Number designation
Any interstate service described in § 64.1501(a)(1)-(2), and not subject to the exclusions contained in § 64.1501(a)(4), shall be offered only through telephone numbers beginning with a 900 service access code.
Any interstate service described in § 64.1501(a)(1)-(2), and not subject to the exclusions contained in § 64.1501(a)(4), shall be offered only through telephone numbers beginning with a 900 service access code.