NGT stands for Next Generation Text and it is the latest incarnation of dear old Typetalk, which dates from the 1990s. I started using TypeTalk in 1995. Later, Typetalk morphed into the Text Relay Service and now (unlike me!) it has moved with the times and is an app.
There is lots of information on the NGT website: http://ngts.org.uk/ and a wonderful NGT lady answered my emailed queries.
I discovered that NGT Lite can be used on a desktop computer, running Windows, Mac OSX, or Linux. It can be used on a tablet running iOS or android. But best of all, it can be used on a mobile phone running iOS or android
I viewed several informative videos on You Tube: https://www.youtube.com/channel/UCH550p9RBdbdvQg0a4HL4cg
It really was a no-brainer: NGT Lite is the way to go.