Deaf-Blind Variation Services

Telebraille also exists for Deaf and/or Blind users with the use of a TTY with a braille keyboard and a refreshable braille display. A relay call by a deaf blind user is directly related to a TTY to Voice call, however often the text transmission speed is reduced to increase the Deaf-Blind user's ability to comprehend the moving Braille on the Braille TTY.

Other variations

Many other call type variations are possible, including VCO to VCO, HCO to HCO, HCO to TTY, and VCO to TTY. Fundamentally, relay services have the ability to connect any two callers with different abilities over the telephone network.

Source: http://en.wikipedia.org/wiki/Telecommunications_Relay_Service#DeafBlind_variation