如何通过Quectel EC21 GSM/LTE模块发送跨运营商短信？

I’ve run into similar cross-carrier SMS issues with Quectel modules before, so let’s break down actionable steps to get this sorted for your EC21:

• Verify the phone number format
  Inter-operator messaging almost always requires the number to use the international E.164 format (with country code prefix) instead of a local short number. For example, domestic Chinese numbers should be written as +8613xxxxxxxxx instead of just 13xxxxxxxxx. Test with this command sequence:

  AT+CMGF=1  // Switch to TEXT mode first
  AT+CMGS="+8613xxxxxxxxx"
  > Test cross-carrier SMS  // Type your message, then press Ctrl+Z to send
  

• Check and correct the SMS Center (SMSC) number
  Your SIM card’s SMSC must be set to your carrier’s official, unrestricted number—using an SMSC limited to intra-operator traffic will block cross-network sends.

  • Check current SMSC configuration: AT+CSCA?
  • If incorrect, set the official SMSC for your carrier. Examples for Chinese carriers:
    • China Mobile: AT+CSCA="+8613800[your-area-code]500"
    • China Unicom: AT+CSCA="+8613010[your-area-code]500"
    • China Telecom: AT+CSCA="+8613300[your-area-code]500"
  • Confirm the change with AT+CSCA? again.

• Switch to PDU mode (if TEXT mode fails)
  Some carriers have better compatibility with PDU mode for inter-operator SMS. Try switching and testing with a simple message:

  AT+CMGF=0  // Switch to PDU mode
  // Example PDU for sending "Test" to +8613xxxxxxxxx (generate your own via online PDU calculators)
  AT+CMGS=17
  > 0011000D916831xxxxxxxxF00008045465374  // Enter the PDU string, then press Ctrl+Z
  

• Adjust SMS encoding settings
  Mismatched encoding can cause silent failures across carriers. For TEXT mode, use the universal UCS2 encoding to support all character types:

  AT+CSMP=17,167,0,8  // Set UCS2 encoding for TEXT mode
  

  Test with a plain English message first—if that works, try Chinese characters next.

• Confirm network registration and signal strength
  Weak signal or incorrect network registration can block cross-carrier sends:

  • Check registration status: AT+CREG? (look for +CREG: 0,1 or +CREG: 0,5, which mean successful registration)
  • Check signal strength: AT+CSQ (a value between 10-31 is good; values below 10 indicate weak signal)
    If signal is poor, move to a better location or adjust your module’s antenna.

• Rule out carrier-side restrictions
  It’s possible your SIM card has restricted inter-operator SMS access. Reach out to your carrier’s support to confirm:

  • Does your plan allow cross-carrier SMS sending?
  • Are there any service blocks or limitations on your SIM card?

• Share the specific error code
  When sending fails, the module returns a +CMS ERROR: [code]—this is critical for pinpointing the exact issue. For example:

  • +CMS ERROR: 310 = Invalid SMSC number
  • +CMS ERROR: 21 = Network rejected the request
  • +CMS ERROR: 500 = Unknown error
    Posting this code will help narrow down the problem much faster.

内容的提问来源于stack exchange，提问作者Ali Sam