This document defines the "Bearer" authentication scheme for the Session Initiation Protocol (SIP) and a mechanism by which user authentication and SIP registration authorization is delegated to a third party, using the OAuth 2.0 framework and OpenID Connect Core 1.0. This document updates RFC 3261 to provide guidance on how a SIP User Agent Client (UAC) responds to a SIP 401/407 response that contains multiple WWW-Authenticate/Proxy-Authenticate header fields.