Sprint CDM-120 Phone Manual page 63

Note:
In TTY Mode, your phone will display the TTY access icon.
Note:
If TTY mode is enabled, the audio quality of non-TTY devices
connected to the headset jack may be impaired.
WARNING: 911 Emergency Calling
Sprint recommends thatTTY 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 51