The phone app on the OnePlus smartphone supports TTY mode, which allows to use text-to-voice or voice-to-text if the feature has been enabled in the app's accessibility settings.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Phone app
Step 2:  Open the  menu
Step 3:  Open the  Settings
Step 4:  Tap on  Accessibility
Step 5:  Tap on  TTY mode
Step 6:  Enable or disable  TTY
  1. Open the Phone app
  2. Open the menu
  3. Open the Settings
  4. Tap on Accessibility
  5. Tap on TTY mode
  6. Enable or disable TTY

OnePlus Instructions