Switching to Google Project Fi

My migration to Google Project Fi is complete. Fi is a cellular service plan that uses the T-Mobile and Sprint networks in the USA and others in other countries.

I got T-Mobile to unlock my previous phone, T-Mobile Lumia 435 (Windows Phone 8.1) before transferring service. Since the account was T-Mobile prepaid, there is not a web interface to do it. You:

I decided what to do with my phone numbers. I moved the T-Mobile one to Fi on one google account, and kept my Google Voice number on a separate google account.

Nexus 5X (ordered as part of signing up for Project Fi)

Fi data SIMs acquired for two old phones.

Even though Project Fi is sort of prepaid, they bill you for any data overages rather than cut off your data, and they require you to keep a credit card on file with them (Google Play gift cards do not work as payment for Project Fi). This is a step down from T-Mobile prepaid, who will helpfully throttle/stop your service rather than risk putting you in $10k of debt (a thing that some postpaid plans notoriously make possible). There is a way to set a data limit but it appears to be an Android feature. Data-SIM devices may not support that, there's probably no way to make the limit be per-Fi-account rather than per-SIM, and anyone who hacks or borrows your phone can change the limit to whatever they want.