Find iPhone is done through the Apple servers, not Xfinity. Find My Phone must be enabled on the device. The phone's location is determined any time the device connects to data. There is no way to track via the cellular signal. If a phone is stolen and never connects to data there is no way to locate the phone.
Hello Kanner, the information that (raythompson) shared is correct. Please reach out to Apple if you continue to have issues with your "find my IPhone" feature.