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.
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.
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