Comment by js2

Boost Mobile (under Dish Network), until a few months ago, ran their own custom-built 5G network that covered about 30% of the US population. They built it after the acquisition of Sprint by T-Mobile, in an effort to maintain a fourth nationwide wireless carrier.

Unfortunately Boost/Dish struggled significantly with finances and customer attraction post COVID, largely due to two problems (seamless roaming between their own network and partners’, and more importantly, getting manufacturers like Apple to build compatible phones). When the current president came into the picture, the FCC essentially forced the sale of Dish’s primary spectrum licenses to administration-friendly SpaceX, for future Starlink use.

As of now, they are in the process of moving their customers to AT&T (and possibly a secondary agreement with T-Mobile), but they seem to be maintaining their own network core - that’s likely why they’re able to implement support for this, while AT&T does not.