ipTTY (sometimes referred to as TTY over IP or TTY over VoIP) is engineered to allow TTY communications using existing telephony infrastructure. The program communicates through IP and uses standard session initiation protocol (SIP). ipTTY enables virtually every user on your telephony network to communicate with customer TTY machines or the Text Relay Service (TRS), without the need for expensive analog lines or FXS gateways. Additionally ipTTY supports audio for hearing and voice carry-over and real-time-text via RFC 4103; the future in real time text communication.
More information: http://www.accessaphone.com/products/iptty/