This is also what I've seen as a major gripe. If the phone has cellular connectivity, it should also trigger to keep phone app over anything else. I actually haven't checked what priority level the app is run on by default; if it isn't run at nice level -15 it would be simple to rename the app, create a shell script with the original name, and then have the script turn CPU to max and run the app at -15.

But how to reserve a set amount of RAM that gets only used by the phone app, that I do not know.