I bought the C6 because I read on other platforms that it works with Swisscom.
BUT making and receiving calls is the only thing that works. Without an additional FritzBox, you can't even create a phone book!!! Nothing.

I find the phone itself nicely finished, nicer in design than the Gigasets. The quality of the display is good. Text is easy to read, colours are rich. The feel of the keys is also pleasant. The phone also feels good in the hand. Ringtones like all landline phones....

Conclusion:
Nice, practical phone with FritzBox. I would buy again.
Without FritzBox, no additional functions. I would therefore not buy again.

Unfortunately, the Fritz Box is required to use this telephone. Only this has the DECT integrated and the phone communicates directly with the Fritz Box (i.e. the answering machine is also located there).
However, it is usually a very good combination, even if the routers are now somewhat outdated (the new ones with Wifi 6 are still not on sale as of March 2020).

With the Sunrise modem I had problems again and again (calls neither went in nor out), since switching to fritzbox no more problems...