RTT that has to work with other RTT systems supports the relevant mechanism: ITU-T V.18 at a PSTN interface, IETF RFC 4103 for SIP-based VoIP (with the 3GPP IMS specifications), a published common specification with lost-character signalling for other technologies, or a newer RTT standard used by all other voice-and-text ICT in that environment.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.