If you want coverage in the boonies, you need a satellite phone. There
are two providers. The units are bulky and the time is expensive. It
doesn't replace a cell phone. Basically you just buy it and keep it in
the plane for emergencies. But it works ANYWHERE there is a view of the
sky. One of the services has a northerly limit, but it is north of the
artic circle. The other even works at the north pole (south pole too).

Cell phone coverage varies a bit from area to area, some companies are
stronger in some locations. AT&T used to have the best coverage,
Verizon seems to be the best now, but check the coverage in the areas
where YOU fly.