Ghost Porting Issue? Newly transferred phone line from ATT to Xfinity now active and being billed by both carriers!!!

I switched a phone line from ATT to Xfinity a week ago and was assured that everything was completed in the Xfinity store.  We even verified the phone line was transferred and active on the Xfinity mobile network via physical SIM card into an iPhone X.  However, the old iPhone 7 with physical SIM is still somehow active and working on the ATT network.  I inquired with ATT about the issue and they said they have not received the port out signal, while Xfinity insists the port out signal was already sent.  ATT has also stated that if they terminate service then I will lose the phone number.  As a result, I now have 2 separate phones active with the exact same phone number under 2 different carriers and I am being billed twice for the same line! HELP!!!!