What we see is that we get a reject reason that points to the traffic not being encrypted.

Did you make an upgrade of the Fritz box when this happened?

Have you installed any new DECT bases?

Have you changed anything in the Fritzbox-config?

It is never a good thing to have the DECT traffic unencrypted. For testing purposes, you can disable the dect encryption to see of that solved the problem. But I would recommend checking the DECt encryption featured from the Fritzbox as well. Different DECT bases can give a problem when one supports encryption and one doesn't for example.

You can disable dect encryption check like this:

Go into the service menu by pressing MENU, long press (2 seconds) on HOLD, long press (2 seconds) on Connectivity button.*
Now go to Dect encr. check and select disabled.
Now try making a call again.
Remember, this is not reccomended to disable in the long run. Check the Fritz box features if this works as a workaround.