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 might also cause audio quality issues.
By default, TTY mode is turned off. You can turn on TTY mode by clicking Phones in the Lync - Options dialog box. You will then need to sign out of Lync 2010 and sign back in to use the feature.