Google Fi App is now in the Apple App Store. It supports activation, MMS configuration, etc. Not sure that you can access Sprint or US Cellular. As a matter of fact, I would tend to doubt it.

https://itunes.apple.com/us/app/goog...413936031?mt=8

Here is a little more information:

https://fi.google.com/luna?p=features

Stu

PS: I also bought a Pixel because their gift card offer made it was effectively free.