The SIM card can easily be inserted and removed (the slot is accessible from the outside and not hidden somewhere inside), so you can easily move it back and forth between the mobile phone and the FritzBox.
I am not aware that you can make phone calls with the SIM card. As far as I have seen, the FritzBox presents itself as a "normal" telephone exchange, regardless of how the Internet connection was established (DSL, WAN port or 4G). To make a phone call, you need a landline or VoIP account and either a "normal" phone (connected by cable or DECT), a VoIP phone or a VoIP client on your mobile or computer.
I would ask the question to AVM support, they are very helpful and may be able to clarify this.