It won't. The cell phone needs the internet to load the map tiles. For navigating with no cell service you need a GPS device with the maps built into it's memory.
I spend probably 2 days a week in the mountains of VA and WV with no cell service. I have a Garmin GPS that mounts to the windshield with a suction cup. I think it cost about $300. I move it between our 3 vehicles depending on what I'm driving. I've even taken it on my boat to find an address by water. I just can't justify the huge cost of a built in navigation system, particularly when it only works for one vehicle.