Real-Time Text (RTT)

RTT lets you transmit audio as you type text, and your recipients can immediately read the text without delay. If you have hearing or speech difficulties, you can use RTT to easily communicate on the phone without special devices. You can make and receive RTT calls on your iPhone and other Apple devices (this feature is not available in all regions or supported by all carriers).

For more information about RTT, see the Apple Support article Make and receive RTT calls on your Mac.