The Fritzbox has no builtin sync functionality to a gereric Carddav service
It would be nice to snc the fritzbox phone books autoimatically with a generic Webdav provide. Unfortunately this connection exist only for google gmx and dome other providers.
Some months ago I raised a feature request for this functionality but I don’t hold my breath
For the time I see only an offline synchronization as workable. This can be implemented with several existing scripts.
I chose https://github.com/pamapa/python-fritzbox to implement a manual workflow to fetch the phone books from a cremote CARDDAV-Server (nextcloud) .
Then I replaced the phonebooks in the fritztbox manually with the downloaded data
I chose The WLS subsystem of windows 10 due to pragmatic reasons and prepared it with
sudo apt update sudo apt upgrade sudo api install git sudo apt install python3-pip sudo pip3 install requests
Then I followed the README of python-fritzbox for the installation, but substituted python3 for python
Because I planned to manually load the phonebooks anyway I did not bother with the certificates.
I copied the URLs of the CALDAV adressbook from the Settigs panel in my nextcloud instance and prepared a special app password for the synchronization. Then I created statements similar to the following an pasted them in a script:
fritzboxphonebook.py --country-code +49 \ --webdav-username <myuser> --webdav-password <mypassword> \ --webdav-url https://xxx.de/remote.php/dav/addressbooks/users/<myuser>/<bookname>/ --save bookname.xml
Note the parameter country-code . It amends incomplete telephone numbers and should be your home country.
I prefer to have sevral phonebooks in the fritzbox and assign them to the different phones.
I open the fritzbox web console, create a new phonebook and restore ist content from the file which I produced.