@Fellfrosch, could you please install https://openrepos.net/content/slava/ofono-logger and send the logs (first review and edit in case there is sensitive data like numbers there) if the SIM is gone again? It should at least give some hints what to look at.
[nemo@Sailfish ~]$ gdbus call -e -d org.nemomobile.voicecall -o / -m org.nemomobile.voicecall.VoiceCallManager.dial '+33611111111' Error: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such method 'dial' in interface 'org.nemomobile.voicecall.VoiceCallManager' at object path '/' (signature 's')