Google's Project Fi is a great cell-phone service, but the data-only SIMs make it incredible for network projects!
I have a lot of cell phones. I have iPhones (old and new), Android phones (old, new, very old and funny-shaped), and I have a few legacy phones that aren't either Android or iPhone. Remember Maemo? Yeah, and I still have one of those old Nokia phones somewhere too. Admittedly, part of the reason I have such a collection is that I tend to hoard nostalgic technology, but part of it is practical too.