AT commands are used for communicating with a modem. AT is the abbreviation of ATtention commands. Every command start with AT. Some of the useful AT commands are listed below. You can call a number, attend a call, hang a call, send sms, read sms, and the list goes on with the features of a particular provider. Some commands are specific to some models. GSM modems can be bought and can be integrated to Micro-controllers or directly to PC. Following list may be helpful to hobbyist or others working on GSM modems for project works.
The response will be OK. This means that the modem is up and ready.ATD94xxxxxxxx
This command will initiate a voice call to the no 94xxxxxxxx. The semicolon indicates that it is a voice call. If you want to run any USSD codes like *123# for balance enquirey, do the same without semicolon.ATD*123#
Other Call control commands
Hang up call
Sending an SMS.
First put the modem into text mode. Otherwise output will be in hex format.
ThenAT+CMGS="+94xxxxxxxx" > This is the text message
Then press Cntrl+z to complete typing message and send it. You will get a response like
+CMGS: 100 OK
Reading a messageAT+CMGR=<indexofmessage>
The response will be the message indexed at 1.
Some other AT commands :
For a whole set of AT commands refer this link..Link
For a quick overview of some useful commands go to this site.Link
For how to communicate (like sending message, calling etc) using a microcontroller and a GSM modem (like SIM300) am gonna write a post soon.