Cell phones in GNP are generally worthless. I discovered yesterday that Verizon (not AT&T) works when you are about 3/4 of the way to Granite Park Chalet. It also works on the Garden Wall Trail and at the Swiftcurrent Lookout. No phones work in Many Glacier or Two Medicine but Verizon does work just before the entrance station at Two Medicine. Before AT&T gobbled up the company that I had been with for 20 years my phone would at least work in St Mary. After AT&T it no longer worked there and had a much smaller footprint in the West Glacier area.
Both AT&T and Verizon work on the Apgar Lookout Trail.
A SPOT works almost everywhere but many Rangers are fed up with them due to the number of frivolous calls that they get.
montana slow hiker