You could build your own. I'm sure you've seen the Arduino DIY phone? Take something like that - but use a RasPi Zero W - add on a better touchscreen (plenty o

You could build your own. I'm sure you've seen the Arduino DIY phone? Take something like that - but use a RasPi Zero W - add on a better touchscreen (plenty of options there). The Arduino version used a 3G module - but if you are willing to fork out the money, you can find 4G modules out there (they aren't cheap, though). Then all you have to do as the code magic, a 3d printed case, etc... You'll still need a cell service provider, of course - no real way to get around that, unless you wanted to set up some kind of wifi auto-hacking system to scrounge off of insecure (and other free) wifi nodes (coverage would be spotty, of course).