Nexus 6P Not Working On AT&T

Nexus 6P unlocked, unbranded Bought from Google brand new in USA Works with all sim cards i have tried: Swisscom, t mobile germany, vodafone portugal, Rostelecom Russia, T Mobile USA, With ATT recognizes carrier But cannot send or receive calls Even to dial help 611, ends with no mobile network Does ATT have a conspiracy to disallow certain models, brands? I tried with 3 different ATT cards from ATT I did even read in ATT forum that this phone used to work.

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