National Relay Service (NRS)

The NRS 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 the NRS to easily communicate on the phone without special devices. You can make and receive NRS calls on your iPhone and other Apple devices (this feature is not available in all regions or supported by all service providers).

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