Text telephone (TTY) mode is used for text communication over a telephone line. A TTY device must be connected to the computer to interpret the modified audio; otherwise you might experience poor audio quality during the call. Using TTY mode in a conference call may also cause audio quality issues.
TTY mode is turned off by default, to turn on the feature:
Connect a TTY device to your computer.
Click Turn on TTY mode, under Phone accessibility.
Sign out of Lync, and then sign back in.