not a great idea you have to have cell phone coverage
You download & store the maps onto your phone's memory, no cell reception is required, unless you want to download maps while on the trail.
I've always downloaded the maps ahead of time, and either downloaded KML trail files, or made my own trail "markers". Much quicker to do this when the phone has a WiFi connection.
If you do a YouTube search for Gaia or BCN, you'll see how this is all done.
I've run Moab, Mojave Road, and some National forests now. Very happy with the perfomance, and file size.