Deaf and hearing individuals can communicate by telephone through a sign language interpreter by placing a video relay call. Video relay calls are made using a high-speed or broadband Internet connection (i.e. DSL, cable, or T1 line) and a videophone connected to a TV, or through a personal computer equipped with a Web camera and video relay software. The person who is deaf signs to a video interpreter on their monitor, who then communicates with a hearing person via a standard phone line by relaying the conversation between the two parties. For more information regarding video relay services, see the following resources.
The Federal Communications Commission (FCC) gives an informative overview about how this technology works in its "Consumer Facts Sheet: Video Relay Services" at http://www.fcc.gov/cgb/consumerfacts/videorelay.html