[Asterisk-Users] TDD/TTY - How does one use this?

[Don Fanning]

I'm trying to look for documentation on how the TDD/TTY interfaces with
the user.  From the looks of it, fskmodem talks directly to a channel.
Does it matter what type of channel it connects to?  SIP/IAX/Zap?
Secondly, how does one interface with it on the asterisk side?
Obviously there is no sendtty function in the cli and it would be the
wrong place for it.  How does it work?

Thanks
-Don