Change Accessibility RTT preferences on Mac
On your Mac, use the RTT pane of Accessibility System Preferences to set up your Mac to make and receive Real-Time Text (RTT) calls.
To change these preferences, choose Apple menu 
> System Preferences, click Accessibility, then click RTT.
Note: These preferences are shown only when you have set up your iPhone and Mac for Wi-Fi calling and RTT phone calls are enabled on your iPhone. RTT is not available in all regions or supported by all carriers.
Enable RTT | Make and receive RTT calls on your Mac without using a special device. When this option is selected and you make a phone call on your Mac, you can choose to use audio, RTT, or RTT relay for the call. |
Send Immediately | Send characters as you type them. To type full messages before you send them, deselect this option. |
RTT relay number | The relay service to use when you make an RTT relay call on your Mac. If a number isn’t specified, your carrier’s default relay service is used. |
When RTT phone calls are enabled on your Mac, the RTT button 
is shown—for example, in a card in the Contacts app or in FaceTime—to indicate that you can make an RTT call. Depending on your network environment, an RTT call may be downgraded to a TTY call.
Important: When making an emergency call in the U.S. on your Mac, special characters or tones are sent to alert the operator. The operator’s ability to receive or respond to these tones can vary depending on your location. Apple doesn’t guarantee that the operator will be able to receive or respond to an RTT call.