It is called a Video Relay Service (VRS) and it is a service for the Deaf community. You usually have a physical VP (Video Phone) or just a VP program on a pc/laptop/smart phone/tablet etc. that has a webcam, and you get your own phone number. The VP can be used to call hearing people with the use of an interpreter (VRS), and although you can call any number, it is mostly used just for calling landlines and business phones (which you cannot text from a cell phone), also it is used to dial 911 (most areas you cannot text 911). The other feature of the VP is for a Deaf person to Deaf person call, with direct visual contact (no interpreter present). Just like if you were to Skype/ooVoo a friend. But instead of using auser namee you have an actual phone number to call each other at (that you also use when calling hearing people & businesses).
If you are Deaf/hh/speech-impaired (or have a child in your household who is), and you use ASL as your main language of communication, you can get the services and products at little or no cost to you (depending on what you choose). If you are hearing you might be able to purchase one at a very expensive cost and pay a monthly/annual fee (even if you don't use it), since it is intended for the Deaf community.
It's illegal to take them at free or discounted prices for Deaf/hh people, because the government puts out that money for the products and for interpreters/employees to work, that's how the companies make money (from the government) since the Deaf community pays little to nothing.
Some companies do sell them for hearing people. I know "ZVRS" (which is one of the best VRS companies - imo), plus it is Deaf owned (which I will always support any Deaf owned business, especially if it's for the Deaf community)... well they have some pretty cool ones. They are portable unlike Sorenson's VP-200, (Sorenson is just another big VRS company) and they can connect to your TV too for large viewing (just like Sorenson, but not like a land-line). ---- Sorry that paragraph was obviously very biased...
The fees are well worth it in my opinion though: I have one (& I pay all the fees too) since I have many Deaf friends that I like to chat with occasionally (even though we just text for the most part), and my bf is deaf too, so we do what we call "VP dates". Plus I babysit some kids (Deaf & hearing) and when it is a good tool to have them practice and learn on. I always make the kids order the pizza by themselves & let them help me make business calls, just as a learning experience, so that they all know how to make proper business calls one day since so many kids these days (Deaf an hearing) have such poor phone skills. And sometimes they will just "hangout" with other signing friends who have VP's at home... kind of like a phone call, it keeps the kids busy for a while and is more interactive than watching tv or playing on the computer.
Although I recomend hearing people who sign to actually support the companies by paying, you won't get shot down if you, as a hearing person, illegally download the free pc (computer) vrs/vp programs and don't use the interpreter... But the government will come knocking on your door if they catch a hearing person taking advantage (without paying full price) of their products specifically paid for and designed for Deaf customers (with fancy features like tv hook up, portable wifi, vibrating/flashing lights notifiers, etc.) That is a convenience for Deaf people, it helps them call 911 if there is an emergency (since you cannot text 911 in all areas) even if they are not home, it also help them make business calls at work and some use the interpreter directly like if they are at a doctors appointment or meeting an accountant. It is like a virtual interpreter as opposed to a physical person interpreter (which they should still stay in business and be hired, but desperate situations arise sometimes for non-oral Deafies, where you cannot wait for an interpreter and would prefer easy communication over writing on paper or playing charades).
Good Luck! Hope I've helped!
Involved in the wonderfully amazing Deaf community :)