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
TTY mode -  Open the  Phone app
TTY mode -  Open the  menu
TTY mode -  Open the  Settings
TTY mode -  Tap on  Accessibility
TTY mode -  Tap on  TTY mode
TTY mode -  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