Understanding
Ok, my understanding of this is pretty simple.
If you are a Deaf person, who does not speak and uses Sign Language, and you want to call a Hearing person, you can use your Videophone or Webcam to call VRS to call a Hearing person. The Deaf person signs, and the interpreter voices. When the Hearing person voices, the interpreter signs.
If you are a Hard-of-Hearing, Latened Deafened, or Oral Deaf person, who speaks and uses Sign Language, and you want to call a Hearing person, you can do the same thing, but you can also ask the interpreter to call you back on your home phone, for VCO. The interpreter will make a 3-way call to the Hearing person, so you can hear whatever you can and also look at the sign language.
The opposite applies if you are a Speech-Disabled person, who does not speak, but can hear, and uses Sign Language, for HCO calls.
If you are a Hearing person, you can call the toll-free number for VRS to reach a videophone/webcam user with a videophone number (Sorenson VRS, Hamilton VRS), ISDN number (Sprint/CSD/Federal VRS), ext. number(Hands On VRS, Sprint/CSD/Federal VRS) or IP address (any VRS provider).
If you are a videophone/webcam user, and you want to call another videophone/webcam user, you do not use VRS. You call them directly, and you use Sign Language. (At this time, there is no way for video conference calling. For example, if a Hard-of-Hearing person, who knows sign, but prefers to speak, wants to call a Deaf person, who uses sign, with an interpreter, that is not possible, without the Deaf person having 2 videophones or a videophone and a webcam.)
If you are Deaf, Hard-of-Hearing, or Speech-Disabled person, who does not use sign, and you want to call a Hearing person, you use TRS or IP-Relay. The same applies if you want to call a Hard-of-Hearing or Speech-Disabled person.
If you are a Deaf person, who does not sign, and you want to call another Deaf person, who does not sign, you do not use relay for that. You call the Deaf person's TTY/ASCII/Telebraille or IM that Deaf person directly.
If you are a Deaf person, who does not sign wanting to call a Deaf person who does sign or the other way around, you are not supposed to use relay for that, according to some. Every VRS, except Sorenson, allow neither videophone/webcam users to call TTY/ASCII/Telebraille/IM/VCO/HCO/STS users, nor TTY/ASCII/Telebraille/IM/VCO/HCO/STS users to call videophone webcam users. Sorenson VRS allows their customers to make VRS-to-Relay calls and Relay-to-VRS calls, because their customer service department claims, "The FCC allows that." However, a Hands On VRS Deaf Customer Service Representative told me, "No, the FCC does not allow that. Sorenson will do anything to they can to increase their number of calls." For example, if a Deaf videophone user, who signs, wants to call a Deaf TTY user, who types French, the Deaf videophone user could tell the interpreter to call Vermont Relay in French, and ask for English-to-French, to type to the Deaf person.