That's why T-Mobile's on-call engineers carry around AT&T phones.
(source: friend who's an engineer at T-Mobile)
(source: friend who's an engineer at T-Mobile)
But do AT+T engineers carry T-Mobile phones?
If yes then they should put themselves together a deal so that none of the on-call engineers have to worry about running up big bills using their phones. When there are freak weather events they are all in it together.