Carriers (at least of the non-MVNO kind) are far too burdened by the transnational legal frameworks of radio regulation to ever really be global. And various legal regulations on companies around the world don’t make it easier for telecommunications providers to form certain kinds of business arrangements.

The best we will ever get is if some MVNO gets a good enough deal on their global transit that they can provide some kind of global roaming service like that by way of agreements with physical carriers in many countries.