I wonder how much DECT radio signal these things need to work. The FritzBox is in the basement, the signal has to go up to the second floor through three thick ceilings. It's not enough to make phone calls on the second floor. WiFi is available everywhere via cable and three access points. The alternative would be a DECT repeater (just Schuko with adapter), which I have seen. I also have a second FritzBox that could possibly take over.