Hello,
you can find full detail of AT+CNMI command in this pdf https://source.sierrawireless.com/resources/airprime/software/at-commands-interface-guide-for-firmware-7,-d-,54,-d-,0,-d-,a1/
Download the pdf from the above link. And you can find other SMS related commands also that works on MC module.
Thanks & Regards,
Rex