TTY (Teletype) and the later term TDD (Telecommunications Device for the Deaf) is a technology used by the hearing impaired to communicate over the phone. In the USA support of TTY is required in some public places (call centers, hotels, etc).

Asterisk has some basic Alpha level code (as of December 2006) with support for TTY.
There is some additional information available at tdd mode. Asterisk svn trunk
has support for T.140 Real-time text and improvements on the TDD mode
in zaptel. TDD support is being developed in SIP too as well as a TDD gateway between
zaptel (analog devices) and SIP/T.140 realtime text in RTP.

See Also

• ipTTY is a good example of an application that supports TTY over IP Product Site
• Network World article on VOIP and TTY (2002)
• Wikipedia TTY article
• FACT OR FICTION - TTY WORKS OVER VOICE OVER IP Cisco conference paper in 2004
• TTY & TTD Over VoIP: Dispelling the "Packet Loss" Myth - Cisco
• TDD/TTY - Information at Assistive Technology.