Multiple communication systems have been invented for people who are deaf or hard of hearing. You may have heard of a few, such as TTY, where a hearing impaired person types their message and it is then read by an operator to the hearing person on the other end. Similar systems exist with variations depending on the users preference, like the Voice Carry Over, the Hearing Carry Over, or the Speech to Speech.
Video Relay Service (VRS) is a service that uses live video to capture its clients using sign language to communicate on the phone. For example, if you were to call someone who uses sign language through VRS, you would dial a special number, get connected with an interpreter and tell them what you wish to say to the person on the other line. The interpreter then communicates with your friend via webcam by signing your message to them. The person signs back a message, which the interpreter then relays to you.
This type of relay service is very convenient in allowing sign language users to communicate the way they normally do when having a conversation with someone. All you need is a computer, a high speed internet connection, and a webcam; the service itself is free.
Visit TDI-Online for a list of U.S. VRS service providers.