Xfinity mobile billing issues—crazy poor

How do I send a message to Xfinity mobile corporate office directly regarding a billing issue that has taken hours and hours to resolve unsuccessfully?  I’m ready to transfer to a new carrier after this experience. I originally had an upgraded one of our iPhone 13s to an iPhone 16, then changed my mind and asked to return the 16 for the 17. I was never credited for returning the iPhone 16 and kept getting charged for the iPhone 16 even though customer service says it seems the IMIE for the phone I returned is showing up as in the xfinity warehouse.