Setting up FRITZ!App FonFRITZ!App Fon allows you to conveniently make telephone calls over the FRITZ!Box with your smartphone when you are at home – the smartphone functions as an IP telephone. You can also use FRITZ!App Fon with several FRITZ!Boxes and thus make calls over different FRITZ!Boxes. For this you must set up FRITZ!App Fon once on each FRITZ!Box as described here. After setting it up,...
> https://en.avm.de/service/knowledge-base/dok/FRITZ-Box-7690/251_Setting-up-FRITZ-App-Fon/
Requirements for HD telephonyWith HD (High Definition) telephony, you will experience natural sound quality during calls. When using HD technology to make a call, the full frequency spectrum of speech is transmitted, resulting in realistic voice reproduction. To conduct calls in HD quality, a number of requirements must be met: The telephone call must be made entirely over the internet or an ISDN...
> https://en.avm.de/service/knowledge-base/dok/FRITZ-Box-7690/323_Requirements-for-HD-telephony/