The Treasury Inspector General for Tax Administration has released Toll-Free Telephone Access Exceeded Expectations, but Access for Hearing- and Speech-Impaired Taxpayers Could Be Improved (2010-40-108):

During the 2010 Filing Season, the IRS exceeded its key toll-free telephone assistance performance measurement goals. However, hearing- and speech-impaired callers that used the IRS Text Tele-typewriter/Telecommunications Device for the Deaf (TTY/TDD) telephone line experienced a low Level of Service and had difficulty reaching an IRS assistor. …The Level of Service for the TTY/TDD toll-free telephone line for the 2010 Filing Season was 8.8%, the lowest Level of Service since the 2003 Filing Season when it was 6.2%. The TTY/TDD product line Level of Service has consistently provided the lowest Level of Service among all of the Customer Account Services Enterprise product lines.