How to use TTY with the Phone app?

[MikeH_yo]

I'm hard of hearing and unable to communicate with a certain government service I need to contact (just can't freaking hear them). They don't support email. But they do have TTY support. I have a vague idea of how TTY works.

So I enabled "full TTY" in the Phone app's settings, figuring that would be all that's needed, but I'm lost. When I tried calling their TTY number, the app just acted as if it was an ordinary phone call (playing the droning, dial-up-like noise you'd expect is actually transmitted). At no point of time did I get to communicate with text as I expected.

Please help. How do I use TTY mode?

 

[mikedt]

I think the idea is you plug your TTY terminal into the phone's headset jack. There's some information about it here:
http://www.ultratec.com/support/ttycell2.php
And the TTY mode on the phone, is for when you're using your TTY terminal for calls, probably because of the signalling. Back in the day with hard-wired phones, TTY used an acoustic coupler.