Hello,
with the FRITZ!Box 6850 5G you can set up the mobile number from the SIM card in the FRITZ!Box:
https://avm.de/service...If you receive an Internet number from your supplier, you can also set this up in the FRITZ!
https://avm.de/service...If you have any further questions or require assistance from our technical support team, please contact us by e-mail at fritzhilft(at)avm(dot)de. Please state the ticket ID #6650345 in your enquiry so that we can allocate your support ticket more quickly.
Kind regards from Berlin
Your AVM team