companies that make or sell TTYs (TDDs)

TTYs or TDDs are also known as text telephones. They assist persons with hearing loss to communicate over standard telephone lines. They can communicate directly with other persons who have TTYs. They can also communicate with people who have voice telephones by using the Telecommunications Relay Service.

See the TTYs (TDDs) discussion in the technology section for general information on TTYs. 

Interested in buying a TTY? Then check the following sources:

Harris Communications