I had paused my service and went to reactivate it yesterday. Nothing worked and was on the phone with fi for an hour before they told me they just learned that tmobile was having issues. Lasted all the way through late this afternoon. From what I can conclude they may use other services but they rely on Tmobile for things like activation of new account, pausing and restarting and now I am thinking also international roaming. There is a reddit thread on this that fi posts to.