The CDC informed me that it was a secret contact, but the mobile phone test report showed negative.

Close contact is not necessarily positive. The CDC informed me that it was a secret contact, but the mobile phone test report showed negative, because secret contact is not necessarily positive. The shorter the close contact time, the lower the chance of infection. Contacts who live together, eat together, go with colleagues and classmates, ride bicycles together, travel together, work together, go to the toilet together, and go to the hospital together should be judged as secret contact.