What Are AT Commands And What Are Functions That Can Be Performed By Using These Commands?

AT commands are used by modems, each AT command have different task, the letter AT is used as a command, it will ask the modem if it ready for taking the commands. If the modem return back as ok, that shows the modem is ready to communicate. If the error massage receives in return that's mean there is an internal modem problem. This problem will rectify to properly working of modem.   AT commands are automatically retrieved by the modem until an additional command is received to modify them, or modem is switched off. Here are the some AT commands and their function.
AT command Function

AT to check Modem on and working ok.
ATA manually answer incoming call
ATDT use to dial the number as tone dial.
ATDP use to dial the number as pulse dial.
ATE1 use to echo the command on Screen
ATE0 use to turn off echo command
ATS0-n waits for (n) of rings before answering the call
AT&F modem resets on default settings
ATZ modem reset by power up defaults
ATH1 use to modem hook off
ATL0 use to set the volume level on 0
ATM1 use to turn on the speaker for dial tone
ATM0 use to turn off the speaker