I recall it "working" on other phones, but it's stuck on T-Mobile or Sprint (depending on your phone's hardware), and you lose out on some functionality since it uses specific bands[0]. Plus you still need an approved phone to activate it.
AFAIK, Fi uses both T-Mobile and Sprint if you have a dual band phone, and wifi if you're in a building without reception. Most phones have been dual band for a couple years now.