47 CFR § 64.2201 - Ringing indication requirements.
(a) A long-distance voice service provider shall not convey a ringing indication to the calling party until the terminating provider has signaled that the called party is being alerted to an incoming call, such as by ringing.
(1) If the terminating provider signals that the called party is being alerted and provides an audio tone or announcement, originating providers must cease any locally generated audible tone or announcement and convey the terminating provider's tone or announcement to the calling party.
(2) The requirements in this paragraph apply to all voice call signaling and transmission technologies and to all long-distance voice service providers, including local exchange carriers as defined in § 64.4001(e), interexchange carriers as defined in § 64.4001(d), providers of commercial mobile radio service as defined in § 20.3 of this chapter, providers of interconnected voice over Internet Protocol (VoIP) service as defined in 47 U.S.C. 153(25), and providers of non-interconnected VoIP service as defined in 47 U.S.C. 153(36) to the extent such providers offer the capability to place calls to or receive calls from the public switched telephone network.
(b)Intermediate providers must return unaltered to providers in the call path any signaling information that indicates that the terminating provider is alerting the called party, such as by ringing.
(1) An intermediate provider may not generate signaling information that indicates the terminating provider is alerting the called party. An intermediate provider must pass the signaling information indicating that the called party is being alerted unaltered to subsequent providers in the call path.
(2)Intermediate providers must also return unaltered any audio tone or announcement provided by the terminating provider.
(3) In this section, the term “intermediate provider” has the same meaning as in § 64.1600(f).
(4) The requirements in this section apply to all voice call signaling and transmission technologies.
(c) The requirements in paragraphs (a) and (b) of this section apply to both interstate and intrastate calls, as well as to both originating and terminating international calls while they are within the United States.