Since A-GPS uses the cell tower to get the list of satellites in view, the GPS on some cellphones will keep working when cell service is lost but won't start working if cell service is unavailable.
I think this means my Samsung doesn't actually have GPS, since fallback to unassisted GPS has never worked for me (yes, I've tried waiting far longer than 15 minutes).
Maybe you can excuse a mental model that doesn't make the GPS vs A-GPS distinction, since A-GPS is often sold as GPS.
[0]https://en.wikipedia.org/wiki/Assisted_GNSS [1]https://news.ycombinator.com/item?id=40042686
