@Kiokoki74 if it still does not work, you have to delete the Zombi phones from the IB, a maximum of 5 DECT phones can be operated on the IB.
You enter this in your browser https://internetbox.swisscom.ch/#telephony/device/settings
and you will be taken to this router website:
My colleague @WalterB has already explained the login mode from the phone.
Here are the complete instructions for your phone:
See page 8: Registering the phone to a base