AT Commands And There Usage

AT commands also called as Attention Commands or Hayes Command
AT commands can be used to collect information from the modem device or the network and can help you troubleshoot modem specific issues. Below are the list of AT commands that most of the Modems uses.

AT Commands	Meaning
AT+CSCA="+651234567"	Set Message Center Number
AT+CGMI	Get the name of manufacturer
AT+GMI	Get the ID of manufacturer
AT+GMM	Get the Model information
AT+CGSN	Get IMEI number (International Mobile Equipment Identity)
AT+CGMR	Get software version
AT+CIMI	Get IMSI number (International Mobile Subscriber Identity)
AT+CNUM	MSISDN
AT+CPAS	Get mobile phone activity status
AT+CREG	Get mobile network registration status
AT+CSQ	Get radio signal strength
AT+CBC	Get battery charge level and battery charging status
ATD, ATA, etc	Establish a data connection or voice connection to a remote modem
ATD, ATA, AT+F*	Send and receive fax
AT+CMGS, AT+CMSS	Send SMS messages
AT+CMGR, AT+CMGL	Read SMS messages
AT+CMGW	Write SMS messages
AT+CMGD	Delete SMS messages
AT+CNMI	Obtain notifications of newly received SMS messages
AT+CPBR	Read phonebook entries
AT+CPBW	Write phonebook entries
AT+CPBF	Search phonebook entries
AT+CLCK	Opening or closing facility locks
AT+CPWD	Changing passwords
AT+CMEE=n	Enable certain error messages
	n= 0 disable +CME ERROR: result code and use ERROR instead.
	n= 1 enable +CME ERROR: result code and use numeric values.
	N= 2 enable +CME ERROR: result code and use verbose values.
AT+COPS=, ,	Change the GSM network  is used to select whether the selection is done automatically by the ME or is forced by this command tooperator given in the format and may be:
	= 0 automatic ( field is ignored).
	= 1 manual.
	= 2 deregister from network.
	= 3 set only .
	= 4 manual / automatic (if manual selection fails, automatic mode is entered).
	:
	= 0 long alphanumeric (default value).
	= 1 short format alphanumeric .
	= 2 numeric .
	string type given in format ; this field may be up to 16 character long for long alphanumeric format, up to 8 characters for short alphanumeric format and 5 characters long for numeric format MCC/MNC codes)
AT+CBST	Change bearer service type
AT+CRLP	Change/configure radio link protocol parameters
AT+CPMS	Storage of SMS messages
AT+CSAS	Save configurations of the GSM/GPRS modem
AT+CRES	Settings related to SMS messaging such as the SMS center address
AT+CMGF=1	Instruct the GSM / GPRS modem to operate in SMS text mode
AT+CMGL="ALL"	List all the messages
AT+CNMA	New message acknowledgement

Check out  this post : "HOW TOs with AT Command"