According to the AP this morning,
Yet the pilots didn't discover their mistake until a flight attendant in the cabin contacted them by intercom, said a source close to the investigation who wasn't authorized to talk publicly and spoke on condition of anonymity. By that time, the plane was over Eau Claire, Wis
Different from the controllers' version, but its possible that the intercom contact then alerted them to other flights, at the direction of ATC, which were calling them on the Denver frequency.
Cabin crew likely wondering why they were not already on the ground at MSP.