You actually want to fetch IMSI number of the SIM. It is a looong number, but first (MSB) 5 or 6 digits will tell you MCC (leading 3 digits) and MNC (following 2 or 3 digits) of SIM card issuing MNO. That’s when IMSI is represented as decimal number. And I don’t know how to determine whether MNC part is 2 or 3 digits (there’s a special flag for this distinction in RRC broadcast messages carrying mobile network identification, which is obviously missing from IMSI).
Try with AT+CIMI … (fetched this from some web site, so I cant vouch for proper operation).
Current network’s MNC can mislead you as device might be roaming in some other network (even if only in “no network” state).