I have been operating my box for many years behind a cable modem, telephony always works perfectly with a total of 3 different providers (Swisscom is not included) and 4 terminals (1x cable, 2x DECT, 1x mobile phone with FritzApp). As always, there are plenty of causes for errors, but your enquiry does not provide any information on this. While LAN/WLAN are almost plug&play, you have to do some configuration for telephony, i.e. you need information from the service provider so that the Fritzbox can log in there, and you have to set up your terminals in the Fritzbox. The Fritzbox is the best thing that can happen to your home network, but if you want plug&play, you have to get your devices pre-configured from the service provider.