Sony SCP-3810 - Manual (Page 54)

42 2B. Settings
To turn TTY Mode on or off:
1. Press > Settings > Others > Accessibility > TTY . 
(You will see an informational message.)
2. Press .
3. Select On to turn TTY mode on.
– or –
Select Off to turn TTY mode off.
Note In TTY Mode, your phone will display the TTY icon if a
TTY device or an optional headset is connected to
your phone.
When enabled, TTY mode may impair the audio
quality of non-TTY devices connected to the headset
jack.
WARNING 911 Emergency Calling 
Sprint recommends that TTY users make
emergency calls by other means, including
Telecommunications Relay Services (TRS),
analog cellular, and landline communications.
Wireless TTY calls to 911 may be corrupted
when received by public safety answering
points (PSAPs), rendering some
communications unintelligible. The problem
encountered appears related to TTY
equipment or software used by PSAPs. This
matter has been brought to the attention of
the FCC, and the wireless industry and the
PSAP community are currently working to
resolve this.