Samsung Galaxy S III User Manual page 280

To turn TTY Mode on or off:
1. From home, tap Apps
2. Tap TTY mode.
3. Tap TTY Full, TTY HCO, or TTY VCO to turn TTY mode on.
– or –
Tap TTY Off to turn TTY mode off.
 Your phone's TTY mode is set.
Settings
> Settings > My device tab > Call.
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