This should work. The better option is to set up the VoIP accounts directly on the Gigaset Box by connecting it to the router via LAN. Here is the manual: https://gse.gigaset.com/fileadmin...
yes, works fine via RJ11 connection at UPC, absolutely no problems, but the box (base station) can be updated with current software (if available) via the RJ45 (LAN) connection without any problems.