I'm so curious about TTY service. Is there any one able to tell how does it run? My friend carries 1 Android phone with his typewriter to the U.S. and ask me how to use the service. Well, I don't know about this totally, except for I know the service is using typing instead of speaking and listening for disabled. How do you use TTY service in common? How do VCO mode and HCO mode work generally? Is it to set your phone with VCO mode and the peer be with HCO mode, and then place the call in between? Or the user must call the call assistant of carrier if he wanna use any of VCO and HCO? I'm curious about the TTY service in everywhere around the world (not only the U.S.). My country has this service as yet. I think this is really a good point to the disabled people. Is there anybody to share any information?