If you plan on rambling in the Scottish Highlands Mountain Rescue would very much rather you took and relied on a paper map. This is not just because mobile coverage is often absent but battery life and its harder to weatherproof a phone than a map (zip top map covers are cheaply available. Us locals often just print them from online of the area being visited and laminate them.
But going into the hills without a paper map and a compass and knowing how to use them in conjunction is dangerous. Oh and a light source that is not your phone as well. The clouds can descend. Having a good idea where you are at all times is useful in not walking off a precipice in the whiteout.