It stands for "text telephone". Dunno where the Y comes into play, but this is what Google tells me lol.
It probably doesn't show up for you bc you don't have it enabled. It's in the accessibility settings for disabled people.
It stands for Teletype a very old text terminal from the 1940s and 50s, often used as terminals to mainframe computers well into the 70s, and by the deaf for much longer. Police departments still support 911 lines with Teletype support for the deaf or mute. Check your local phone book.
That said I still can't find any setting in my international HOX that turns this on.