There are a variety of technologies that are in use by the deaf to communicate over the phone (see: Wikipedia: TDD)

TDD is a phone relay service, which means there's a hearing mediator in the middle who is translating what the phone system says into a written text message and then the callers' written text message into a spoken response. TDD systems are likely to be used by someone without access to or know-how to go online and accomplish her goal, most likely an older adult. The IVR needs to invoke higher-than-usual timeout thresholds to provide enough time for the TDD system to do its job, or even better, make transferring to a live person easy for this caller population.In a nutshell, callers with hearing impairments need more time to respond to the IVR.

As for dedicated TTY lines, most of the time those just go straight to a live person so there is no threat of timing out.