I seem to recall the notice for excess roaming is via Text Message, but it could be email. Hearsay since I have never been warned.

The 5 Gb high speed data award country count depends on the plan of the user. I don't have the specifics at hand, but basically some get 210 countries, some only 11. It seems to be that the higher priced, newer ones get the 210. Older, less expensive were also included with 11 European countries only. The same with the flight WiFi packages. 4 flights per year for older, cheaper plans and free unlimited WiFi, no flight limit for newer, more expensive plans.

My older ONE Plan only gets the lower level 4 flight perk, but since I have an add on feature, (ONE Plus Promo, Grandfathered Promotion) I get the 5Gb data on all my lines. I gotta say, they have treated me pretty good since around 2015 or so when I first hooked up with them mostly for the international roaming service. I get good T-Mobile service just about everywhere I go normally and their customer service has always treated me fairly.