I had no idea how a Teletypewriter or TTY worked until I saw them on payphones. They are communication devices for the deaf, to type messages over phone lines, made obsolete by texting. Basically, when someone types on a TTY it sends signals down a phone line, where another TTY decodes them back into text. Payphone TTYs work the same way, except you have to pay, of course.
I saw payphone TTYs and thought it would be fun to be able to "text" a smartphone through a payphone. The smartphone needs TTY/RTT calling mode on, which allows the smartphone to become a TTY.
To use a payphone TTY:
Dial and pay for call as normal, leave handset off hook. The number you call must be a cell phone with TTY/RTT mode on, or a landline with a TTY unit.
When other person starts typing on their TTY, the drawer automatically opens and turns on.
Start typing, press"GA", press return when your message is done - like a chat room or texting.
When done type "SK", hang up the handset, and the drawer automatically closes.
Traditionally, when messaging through a TTY, after you're done with your one line, you type "GA" which stands for "Go ahead." this lets the other person know it's their turn to start typing. "SK" stands for stop keying, this is sort of a "Goodbye" and the call ends.
Tip: Before making a call, make sure the TTY is working by checking if the signal light turns on when the phone has dial tone.
The only payphone I've confirmed to have a working TTY is the payphone at Montgomery St. BART station. It's dirty, but it does work! The TTYs at 16th, St. Mission, 24th St. Mission, and Glen Park stations don't work, sadly. This payphone's number is (415) 421-9073. I'll be checking more TTYs as time goes on.
On a side note, the keyboard is a completely metal chiclet keyboard, it looks very industrial on its own, but pair it with the backlit liquid crystal display, and it looks straight out of the 1980s.