Re: My HTC one (8) does not display "hearing aid" option after settings/call/..
TTY Settings - The all new HTC One® (M8)
[Device-Specific Instructions]
Note TTY Settings allow Text Telephone devices (TTYs) to be enabled so that deaf and hard of hearing customers can use the telephone by utilizing a visual, instead of auditory, mode of communication.
From a Home screen, tap Apps Apps icon (located at the bottom).
Tap Settings.
Note If unavailable, tap the dropdown menu Applications dropdown menu icon (located in the upper left) then tap Alphabetical. If still unavailable, refer to Manage App Icons.
From the DEVICE section, tap Call.
From the OTHER CALL SETTINGS section, tap CDMA call settings.
Tap TTY mode.
Select the desired TTY mode:
TTY Off
Disables all TTY settings
TTY Full
Enables all TTY settings
TTY HCO
Enables HCO (Hearing Carry Over)
TTY VCO
Enables VCO (Voice Carry Over)