How can I use a mobile broadband stick with a VoIP server to route calls to multiple devices over Wi-Fi and VPN?

How can I manage incoming and outgoing calls directly via a VoIP server with a compatible 5G/4G/3G stick? I want to use a SIM card that offers a flat rate for calls in Europe. The idea is to use it with multiple devices on Wi-Fi and via VPN with a VoIP client. I also want to share the Internet connection via the hotspot function (WLAN/USB). It's okay if the connection drops during a call. I have been looking for a solution for 2 years. Is there any hardware, e.g. an AVM Fritzbox, that redirects mobile calls from a 5G/4G stick to DECT? I haven't found anything suitable so far.