TDD
Stands for Telecommunication Device for the Deaf.
In the U.S., TDD is governed by Title IV of the Americans with Disabilities Act. All telephone companies are required to provide telecommunication relay services by dialing 7-1-1 to reach a telecommunications relay service, abbreviated as TRS, after which an assistant can place a call using the 10-digit number on the caller’s behalf. The caller can then use a TDD to speak to the party on the other end of the call and read the response.
For more information on the laws governing TDD, see the FCC.