Unfortunately, I cannot contribute anything to connecting the Yealink T48G to UPC. I needed Swisscom support to work with the Swisscom router, because Swisscom does not officially support this device. However, I am very satisfied with the Yealink T48G.

This is to be expected, the device can handle normal SIP accounts. Either provision it in your system before commissioning or set up the account manually in the web interface.