Samsung SPH-A640 Phone Manual page 78

Note:
In TTY Mode, your phone will display the TTY access icon
when a headset or TTY device is plugged in.
If TTY mode is enabled, the audio quality of non-TTY
devices connected to the headset jack may be
impaired.
WARNING
56
911 Emergency Calling
Trumpet Mobile 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 PSAP community are currently working
to resolve this.
Section 2B: Controlling Your Phone's Settings