AT commands for gsm shield with audio

arduino_gsm_hello.ino

#include <SoftwareSerial.h>
#include <string.h>

char incoming_char = 0;
SoftwareSerial cell(2, 3);


const char* commands[]={"ATZ", 
"AT", 
"AT+CGREG?",
"AT+IFC=1,1",
"AT+CMGF=1",
"AT+CLIP=1",
"ATE0",
"AT+COLP=1",
"AT+QAUDCH=1"};

String readCell()
{
  String response = "";
  while(cell.available() > 0)
  {
    incoming_char=cell.read();
    
    if((incoming_char >= ' ') && (incoming_char <= 'z'))
      response += incoming_char;
  }

  return response;
}


void runCommand(const char *cmd){
    // send command to modem
    cell.print(cmd);
    cell.print("\r");

    // read response
    String rc = readCell();

    // debugging
    Serial.println(cmd);
    Serial.println(rc);
}

void setup()
{
  Serial.begin(9600);
  cell.begin(9600);
  
  Serial.print("Starting modem communication...");
  delay(12000);
  
  for (int i = 0; i < 9; i++){
      runCommand(commands[i]);
      delay(1500);
  }
  Serial.print("OK\nWaiting for phone call:\n"); 

}

void loop()
{
  String rc = readCell();

  if(rc.startsWith("RING")){
    Serial.println("Answering...");
    delay(1000);
    cell.print("ATA\r");
  }

  // allow AT commands to be read over serial port
  if(Serial.available() > 0)
  {
    incoming_char = Serial.read();
    Serial.print(incoming_char);
    cell.print(incoming_char);
  }

  delay(1000);

  
}