Why would it eventually get to the surface? The issue with submarines is that they don't normally float (and if you need them to have positive buoyancy, they have to be functional).
Because its been programmed to return to the surface upon error, that is what I've read about 30 times over.
You'd just use your phone for GPS. Doing something useful with that information may be a challenge. All that is moot if they are locked on the wrong side of the airtight hatchway.