The Fritz!App Fon should in theory allow a mobile phone to be used as a handset for a landline when using a Fritzbox router (I have the FritzBox 7590).
I have installed the Fritz!App Fon on both Android and iOS phones. However, the Apple version of the app won't ring for an incoming phone call unless the app is currently open on the phone - if you're doing something else it won't ring, which is not very useful. Yes I have tried playing with all the options in the Fritz!App Fon app, but it makes no difference in the outcome. The Android phone just worked as soon as it was installed with no problems.
Has any one had similar problems and could you fix it? I have a support question into Fritz, but I'm not holding my breath. Two Degrees of course don't provide any support for the Fritz!App Fon app, which is not very helpful.