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

"WiFi Calling" is a feature offered by a number of mobile network operators 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.

To use "WiFi Calling", ensure the following:

• If you are using "Vodafone WiFi Calling", use the DNS server in the FRITZ!Box that was assigned by the internet service provider. Using a different DNS server (for example Google Public DNS) can lead to problems.
• If you also want to make "WiFi Calling" available in the guest network: Disable the option "Restrict internet applications: Only surfing and mail allowed" under "Wi-Fi > Guest Access" in the FRITZ!Box user interface.
• If you are using a FRITZ!Repeater or 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 its user interface.