IETF RFC 8373 PDF

Users have various human (i.e., natural) language needs, abilities, and preferences regarding spoken, written, and signed languages. This document defines new Session Description Protocol (SDP) medialevel attributes so that when establishing interactive communication sessions ("calls"), it is possible to negotiate (i.e., communicate and match) the caller’s language and media needs with the capabilities of the called party. This is especially important for emergency calls, because it allows for a call to be handled by a call taker capable of communicating with the user or for a translator or relay operator to be bridged into the call during setup. However, this also applies to non-emergency calls (for example, calls to a company call center).