Question about AVM FRITZ!Fon C5 International / CH

Hello Zäme
Unfortunately I can't answer this question properly because I also have a FritzBox. However, I assume that the Swisscombox has DECT transmission capability, in which case the phone should work. Before I bought the FritzFon I had a Gigaset phone connected to the Fritzbox. With this phone I could make calls without any problems and also access the address book.
Hopefully this information will help you.

I think so. I have connected my phone to the UPC box and everything works.