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

WiFi calling is a technology offered by a number of mobile network operators that allows their customers to make mobile calls over any wireless network. In order to use "WiFi calling", you only need a compatible mobile telephone and wireless access to the internet.

For reliable operation, we recommend the following:

• When you use your provider's WiFi calling service, do not change the DNS server assigned by the internet service provider (factory setting) in the FRITZ!Box. Using a different DNS server (for example Google Public DNS) can lead to problems.
• If you are using a FRITZ!Repeater or FRITZ!Powerline adapter with wireless capabilities, enable the option "The active wireless devices displayed below may communicate with each other" (factory setting) under "Wireless" ("WLAN") > "Security" in its user interface.