Yes, you can.

I could not connect the phone directly to the UPC. I still needed a base station. I think it's similar with Swisscom

Hello.
According to the Swisscom documentation on this box, it accepts 5 HD phones on an integrated DECT base. So in my opinion it should work without a problem. You should know that the Gigaset Comfort model above must have a DECT station to connect. This appears to be the case with this box. Connection is easy at the touch of a button. Have a nice evening.