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The pixel and Pexel 2 are not T-mobile devices and thus completely controlled by Google. They should theoretically work as they are and if there is anything stoping the roaming it would be on Google for disabling it not T-mobile.
It's unfortunate that the VoLTE capabilities of various phones are somewhat opaque to the consumer. There was a list of phones that would be able to roam USCC but why some made the list while others didn't was a mystery. AFAIK, the list has not been updated and I don't know to what extent the USCC roaming agreement is even still extant.