You can easily use WiFi Calling with the FRITZ!Box.

WiFi Calling is a feature offered by a number of mobile network operators (for example Vodafone) that allows their customers to make mobile calls over any Wi-Fi network. To use "WiFi Calling", you only need a compatible mobile telephone and Wi-Fi access to the internet.

For reliable operation, we recommend the following:

• To use "Vodafone WiFi Calling" with the FRITZ!Box, use the DNS server assigned by the internet service provider (factory setting). Using a different DNS server (for example Google Public DNS) can lead to problems.
• If you are using a FRITZ!Repeater or a FRITZ!Powerline adapter with Wi-Fi support , enable the option "The active wireless devices displayed below may communicate with each other" (default setting) under "Wi-Fi > Security" in the user interface.