Microcell Impact on Bill/Minutes Used

I recently exceeded my monthly allotment of minutes but many (most?) billable minutes were via the Microcell. I was under the impression that those minutes would not count since the traffic is offloaded to my internet account (non-ATT) and not on the ATT Wireless network. Was I mistaken? How do microcell minutes count against allotment?

Any insight appreciated,

john