This Standard defines the requirements for the reliable transport, using Text Relay, of Textphones using TIA-825-A signals, and other signaling associated with Baudot TDDs, across IP networks that connect two or more PSTNs (Figure 1). The Standard describes the functionality of Text Telephony Gateways, which connect between the PSTN and an IP network. This Standard also provides the ability for switching between TIA-825-A signals and voice during a call. The Standard defines Text Relay for TIA-825-A to TIA-825-A Textphone connections with Voice Band Data (VBD) being supported only as a means to support connections that include at least one non TIA-825-A terminal or one gateway that does not support TIA-1001 or ITU-T Recommendation V.151.