Text telephone (TTY) mode is used to send text 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. Using TTY mode in a conference call may also cause audio quality issues.
TTY mode is turned off by default. To turn it on:
Connect a TTY device to your computer.
In the LyncMicrosoft Store app, swipe in from the right edge of the screen, and then tap Settings.
(If you’re using a mouse, point to the upper-right corner of the screen, move the mouse pointer down, and then click Settings.)
In the Phone accessibility section, under TTY, move the slider to the right to turn on TTY mode.
Sign out of Lync, and then sign back in.