The module will respond with an error message when sending some SMS commands if SIM card initialization is not complete, so this must be completed before operating SMS commands. In general, SIM card initialization will be complete about 20 seconds after powering on. However, for some SIM cards, it will take about 40 seconds due to different electrical characteristics. You can use AT+QINISTAT to check the status of SMS initialization. +QINISTAT:3  means the SMS initialization is complete.

Note: Before SIM card initialization is complete, executing the following commands will respond error:

QMGDA /CGSMS /CSMS/ CMGL / CSAS/CRES/ CMSS /CSCB/ CMGW /CMGD/ CMGR /CPMS