I get a feeling that the authorities aren't going to let this happen. Here's why:
The merger creates a monopoly on the GSM market. GSM is the cell phone tech that is almost exclusively used everywhere in the world. There are two other common technologies: PHS, common in east Asia, and CDMA, only used in the USA (Verzion and Sprint) and China (Unicom). That means, if anyone from anywhere outside the US roams, they are restricted to roaming on this new "AT&T/T-Mobile monster".
I actually left T-Mobile because I was just tired of the contracts and nonsense, but AT&T is far worse in that regard. In fact, ever since I ended my landline and DSL service with AT&T, they've accounted for 40% of my junk mail and have outright lied to me to try to get me back.
These days, I'm with a MVNO called PagePlus Cellular which resells Verizon. Minutes are like candy: I can stop purchasing them when I don't want any more, and the vendor has no business in knowing any of my personal info; they don't even know my name

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