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SMS messages can't be sent with GSM modem

Question

Why can't I send SMS messages from my GSM modem (Siemens M20, MC35 or TC35)?

Answer

The cause is most likely a defective GSM modem. Therefore, we want to do two tests to confirm this.

The first test is to test the quality of the SIM card to determine whether it's defective or not. Do this by simply removing the SIM card from the modem and inserting it in an ordinary cell phone. Now, send an SMS from the cell phone. If this succeeds, then we can assume that the SIM card is working properly. Now insert the SIM card back into the modem.

The second test to do is to see if the modem itself is in order. We use Windows' Hyper Terminal program to accomplish this.

  1. Make sure that the modem is properly connected to the PC.
  2. Open the Hyperterminal program (found in Start--All Programs--Accessories--Communications--Hyper Terminal) and make sure you've selected the correct COM port in the setup parameters to which you've connected your modem.
  3. Now key in the following commands, which will then be sent to the modem.
  •           ATE1 [ENTER]
  •           AT+CMGF=1 [ENTER]
  •           AT+CMEE=2 [ENTER]
  •           AT+CPIN=xxxx [ENTER] (where xxxx is the four digit PIN code on the SIM card. If no PIN code is used, then do not include this line.)
  •           AT+CMGW=xxxxxxxx [ENTER] (where xxxxxxxx is the eight digit telephone number of the cell phone receiving the SMS.) After hitting [ENTER], then the         greater than (>) character is displayed.
  •            > aabbcc (where aabbcc is the text content of the SMS to be sent) [CTRL+Z].
  •           AT+CMSS=1
Note: After each [ENTER] is keyed in, the OK message is returned indicating success. If at any time the message ERROR appears, then this is an indication of a defective modem. The modem is considered to be free of defects if the SMS is successfully received by the cell phone, whose number was keyed in in the above sequence.