How to use Real Time Text (RTT) on iPhone
Toggle on Send Immediately if you want to send characters as your type them. Otherwise, you'll need to type full messages before you send them. Toggle on Answer All Calls as RTT/TTY so you don't receive voice calls. If not, you can still upgrade any calls that you get by using the RTT button (see below). Confirm by tapping Answer All Calls as RTT/TTY. Toggle on Hardware TTY to answer and make TTY calls from an external TTY device instead of the iPhone. In this case, make sure that you also turn on Answer All Calls as TTY.
Source: imore.com