Receive and answer SIM900 GSM

Hi ! i have SIM900 GSM my arduino project is to receive and answer to the same number (if another phone send SMS to answer him) what command can i use ? i find this AT+CNUM but i dont know how can i make the syntax… is hard for me because my english is bad ! i need command to show me only the number of the phone so as to register the price and to mend applying the answer.

Well I don't know which one you have, but for instance this page has lots of transferable information:

http://www.seeedstudio.com/wiki/GPRS_Shield_V2.0

Thanks for your answer but did not find what I was looking for …

i have buy from ebay this exactly : http://www.ebay.com/itm/SIMCOM-SIM900-Quad-band-GSM-GPRS-Shield-Development-Board-for-Arduino-Iduino-/190722841911?pt=LH_DefaultDomain_0&hash=item2c67f79d37

i have find this tutorial is very useful http://tronixstuff.com/2014/01/08/tutorial-arduino-and-sim900-gsm-modules/

#include <SoftwareSerial.h>
SoftwareSerial SIM900(7, 8);

char incoming_char=0;

void setup()
{
Serial.begin(19200); // for serial monitor
SIM900.begin(19200); // for GSM shield
SIM900power(); // turn on shield
delay(20000); // give time to log on to network.

SIM900.print(“AT+CMGF=1\r”); // set SMS mode to text
delay(100);
SIM900.print(“AT+CNMI=2,2,0,0,0\r”);
// blurt out contents of new SMS upon receipt to the GSM shield’s serial out
delay(100);
}

void SIM900power()
// software equivalent of pressing the GSM shield “power” button
{
digitalWrite(9, HIGH);
delay(1000);
digitalWrite(9, LOW);
delay(7000);
}

void loop()
{
// Now we simply display any text that the GSM shield sends out on the serial monitor
if(SIM900.available() >0)
{
incoming_char=SIM900.read(); //Get the character from the cellular serial port.
Serial.print(incoming_char); //Print the incoming character to the terminal.
}
}

?t this program he can print on display the number of the sender the date and time of the SMS receipt. I want to take the number of the sender to use it for my unswer SMS. How can i do this ? i cant find another command to show only the number and if i find it how can i use it to send it ?

Try this site:

http://m2msupport.net/m2msupport/module-tester/

You can use their java application to operate your GSM module, test, experiment, and learn what capabilities it has.