47 C.F.R. § 9.6
Upon completion of translation and routing of 911 calls to a PSAP, a statewide default answering point, to an appropriate local emergency authority, or, where no PSAP nor statewide default answering point has been designated and no appropriate local emergency authority has been selected by an authorized state or local entity, to an appropriate local emergency authority, identified by a telecommunications carrier based on the exercise of reasonable judgment, the telecommunications carrier shall provide permissive dialing between 911 and any other seven-or ten-digit emergency number or an abbreviated dialing code other than 911 that the public has previously used to reach emergency service providers until the appropriate State or local jurisdiction determines to phase out the use of such seven-or ten-digit number entirely and use 911 exclusively.
47 C.F.R. §9.6