This avoids warnings like
gnome-calls[57992]: mm_sim_get_imsi: assertion 'MM_IS_SIM (self)' failed
gnome-calls[57992]: mm_sim_dup_emergency_numbers: assertion 'MM_IS_SIM (self)' failed
in the logs
Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/735>