I haven't used a TTY (TDD) in decades so don't know for sure. I would think the same rates would apply as it does for a voice call. It may or may not cost a little more since tty calls take longer sometimes. That's assuming for a landline.
For cellphones- there's no charge if you have an unlimited plan. Just need a connection of some sort between phone and TTY.
Didn't mean to sound flippant. Of course I knew the old teletype machines, the kind they had in newsrooms, the ones that looks like it weighed 500 pounds. You couldn't type faster than the machine allowed. Then BOOM, along came the newfangled TTY. (In those days they were called TTY, not TDD. You ended your sentences with ga or at the other end an sk. The old timers here know.
thanks and yeah, now they call them tdd and or tty. the ga and sk , I know all too well. I had a TDD and now I am not sure if I still have it? I try to google the right question came up one short of a FF. :p hahahha