I know there was a time when Google navigation didn't work on roads where there was no signal, which was a big problem in the US, but nowadays you can download maps. I permanently have the whole of South-East england installed on my phone.
That's great and I sometimes use a phone when I'm in the mountains but there is no way I would rely on one even if you can use a map app off line. I have learned to my cost that phones can over heat in hot weather, freeze in cold weather and battery life can fade fast.
I use mine as a quick check but if you are going into a mountain region you should be able to use a map and compass.
I tend to take the relevant map and have that in a side pocket of my ruck sack and a laminated sheet ( or sheets) with a marked out plan of the hike handy in my pocket. I always have a compass in my pocket and a spare one in the rucksack.
The phones are a fantastic invention and have some great mountain apps but I generally just use them for pictures while I'm on the hills.