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!WLAN Repeater or FRITZ!Powerline adapter with wireless capabilities, enable the option "The active wireless devices displayed below may communicate with each other" (factory setting) in its user interface under "Wireless" ("WLAN") > "Security".