I’m not sure if the modem within the Spitz AX could support calling over cellular. If it could then I do not know if it could be setup to integrate with an asterisk (voip) server. If that’s possible then doing all that would give you a voip server, which could be used to make calls.
I know some work by the community was done for the GL-MiFi in this topic: GL-MiFi [EC25-AF] Asterisk Voice channel?