How to send a automatic reply to a number from which the message is received

I was trying to do this but I couldn’t figure out how to read the number from which the message is received .I am able to see the received number on the serial monitor .please some one could help me to complete this .

I couldn't figure out how to read the number from which the message is received

Why not? What DID you try? What actually happened?

I used AT commands to receive the number the message received will be displayed in the serial monitor but I am not able copy the string which I received into an array .could you please help me out and I am using Sim 900 module and arduino uno board