I have been upgraded a few times but the most memorable was the upgrade I got on a domestic flight in Sweden some 20 years ago. This was before 9/11 and when unassigned seating was common. On a winter morning flight from Gothenburg to Stockholm on a BAe146 (a flight I had done many times before),...

The carriers seem to handle this well (sigh!):- Works as described only on post paid plans.- PAYG plans charges extra and require registered SIM-cards (this is a service for EU-citizens BTW).- Maximum roaming data levels (8-10 GB).- Throttled speed when abroad.etc.Your milage may vary. This is wh...