OTP SMS received in EC200U but not visible in mobile after SIM swap — need clarification on SMS delivery behavior

IoT Modules LTE Standard Module
1

Hi Quectel Team,

I am using EC200U with STM32. I send an OTP request to a server via UDP using EC200U. After sending the request, I pause the STM32 (breakpoint), remove the SIM from EC200U, and insert it into a mobile — but no OTP arrives in the mobile.

However, if the SIM is already in the mobile before sending the request, the OTP arrives correctly.

Commands used in EC200U before sending request

AT
ATE0
AT+CRSM=176,28539,0,0,12
AT+CRSM=176,28539,0,0,12,“FFFFFFFFFFFFFFFFFFFFFFFF”
AT+CFUN=1
AT+CMGF=1
AT+CMGD=1,4
AT+CIMI
AT+IFC=0,0
AT+CSMS=1
AT+CSCS=“IRA”
AT+CSMP=17,167,0,241
AT+CPMS=“SM”,“SM”,“SM”
AT+CSDH=1
AT+QSIMSTAT=1
AT+CGREG=2
AT+CTZU=3
AT+CMEE=2

Still, the mobile never shows the SMS when the request is first sent via EC200U.

My questions:

  1. Does EC200U “claim” the SMS at the network level as soon as it is delivered?
  2. Even with CNMI=2,1, is the SMS guaranteed to be stored in SIM before removal?
  3. Is this behavior due to SMS class (e.g., Class-0/Flash SMS) or network policy?
  4. Is there any modem setting that allows the same OTP to be readable in mobile after SIM swap?

My assumption:

Once the OTP is delivered to EC200U, the network does not retry delivery to the mobile, so removing the SIM later is too late.

Please confirm if this understanding is correct.

What I observe

  • When EC200U is powered and registered on the network, the OTP seems to be delivered to EC200U.
  • Once delivered to EC200U, removing the SIM later does not make the same OTP appear in the mobile.
  • It appears that the network does not re-deliver or retry the SMS to the mobile after the SIM is moved.
2

Hi Mohd,

Your understanding is correct. Once the network successfully delivers an SMS to the registered device (in this case, your EC200U module), the SMS Service Center (SMSC) considers the transaction complete and will not retry delivery to another device just because the SIM has been moved.

For the OTP to be visible on your mobile phone after swapping the SIM, it must have been fully received and successfully written to the SIM card’s message storage area by the EC200U before removal

Please refer below for answers to your question:

  1. Does EC200U “claim” the SMS at the network level as soon as it is delivered?
    Yes. Because the EC200U is registered on the network with that SIM’s IMSI, the network targets that specific hardware for delivery. As soon as the module acknowledges the receipt of the SMS to the network, the SMSC stops any further delivery attempts for that specific message

  2. Even with CNMI=2,1, is the SMS guaranteed to be stored in SIM before removal?
    No, it is not guaranteed if the SIM is removed too quickly. When you use AT+CNMI=2,1 , the module is configured to send a +CMTI: “SM”, unsolicited result code (URC) only after the message has been successfully stored in the SIM

  3. Is this behavior due to SMS class (e.g., Class-0/Flash SMS) or network policy?
    Your command AT+CSMP=17,167,0,241 sets the Data Coding Scheme (DCS) to 241 (which is 0xF1 in hex). According to 3GPP specifications, this indicates a Class 1 SMS.
    Class 1 messages are intended to be stored in the memory of the device or the SIM

  4. Is there any modem setting that allows the same OTP to be readable in mobile after SIM swap?
    Your current settings are correct for targeting the SIM:

AT+CPMS=“SM”,“SM”,“SM” correctly sets the preferred storage to the SIM card.
AT+CNMI=2,1 correctly configures the module to notify you once storage is complete