TTY is a way for the deaf to communicate using the phone.
http://en.wikipedia.org/wiki/Telecom...e_for_the_deaf
We had something like this when I worked at Dell. The deaf person calls and an intermediary would type what I said for the deaf person and then tell me what the deaf person typed. It's irritating because it takes forever to communicate through the relay and the protocol would get mixed with the "real" stuff.
You can use any phone when you have an intermediary, but I guess at a pay phone you'd have to have someone with you there. That probably wouldn't work.