Real-Time Text (RTT)
RTT combines voice and typed text during phone calls without needing special devices; this is especially useful if you have hearing or speech difficulties. With RTT, you can type text during a phone call and your recipients can read your text immediately. You can make and receive RTT calls on your iPhone and other Apple devices (this feature is not available in all countries or regions or supported by all network providers).
For more information about RTT, see the Apple Support article Make and receive RTT calls on your Mac.