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T-Mobile as well here and I have never once received a robocall.

I wonder if they are actively trying to do something to prevent this already and now AT&T wants to join?

I would guess it comes down to the old rules about telemarketers not calling cell phones. If you are in a 'pure' cell phone exchange (the 3 digits after the area code), the number is probably excluded from some dialing lists by inertia.

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