Not going to get into Why carriers continue to charge extra for this, but to
enable the built-in hotspot feature on Android Marshmallow (6.0), I had to root
the phone, then add
net.tethering.noprovisioning=true to the end of
/system/build.prop (and reboot).
settings put global tether_dun_required
into a shell is the way to enable it for phones running KitKat, but that didn’t
seem to do anything.