Avaya Communication Manager Administrator's Manual page 1099

TDD/TTY Mode
Valid entries
off
US
UK
pass-through
Usage
Turn off special TTY handling when using this codec set.
In this case, the TTY transmission is treated like an
ordinary voice call.
With a codec set that uses G.711, this setting is required
to send TTY calls to non-Avaya systems. However, there
may be errors in character transmissions.
For users in regions using this codec, use U.S. Baudot
45.45 mode for TTY transmissions over IP network
facilities. This is the default for new installations and
upgrades to Communication Manager R2.1.
For users in regions using this codec, use U.K. Baudot 50
mode for TTY transmissions over IP network facilities.
For users in regions using this codec, use pass-through
mode for TTY transmissions over IP network facilities.
IP Codec Set
Issue 1 June 2005 1099