Real-time text meant to interwork with other systems uses the applicable mechanism: ITU-T V.18 on the PSTN, IETF RFC 4103 for SIP voice over IP (the 3GPP IMS specifications applying), for other technologies an openly published specification able to flag lost or corrupted characters for other technologies, or a later RTT standard all other voice-and-text ICT in that environment supports.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.