arduino gsm shield error 0 - arduino

Question

I have been trying to send a simple message using an arduino microcontroller and a gsm shield but it keeps getting an error 0.
Here is the code:
int8_t answer;
int onModulePin= 2;
char aux_string[20];
char phone_number[]="*********";
void setup(){
pinMode(onModulePin, OUTPUT);
Serial.begin(115200);
Serial.println("Starting...");
power_on();
delay(3000);
// sets the PIN code
sendATcommand("AT+CPIN=****", "OK", 2000);
delay(3000);
Serial.println("Connecting to the network...");
while( (sendATcommand("AT+CREG?", "+CREG: 0,1", 500) || sendATcommand("AT+CREG?", "+CREG: 0,5", 500)) == 0 );
Serial.print("Setting SMS mode...");
sendATcommand("AT+CMGF=1", "OK", 1000); // sets the SMS mode to text
Serial.println("Sending SMS");
sprintf(aux_string,"AT+CMGS=\"%s\"", phone_number);
answer = sendATcommand(aux_string, ">", 2000); // send the SMS number
if (answer == 1)
{
Serial.println("Test-Arduino-Hello World");
Serial.write(0x1A);
answer = sendATcommand("", "OK", 20000);
if (answer == 1)
{
Serial.print("Sent ");
}
else
{
Serial.print("error ");
}
}
else
{
Serial.print("error ");
Serial.println(answer, DEC);
}
}
void loop(){
}
void power_on(){
uint8_t answer=0;
// checks if the module is started
answer = sendATcommand("AT", "OK", 2000);
if (answer == 0)
{
// power on pulse
digitalWrite(onModulePin,HIGH);
delay(3000);
digitalWrite(onModulePin,LOW);
// waits for an answer from the module
while(answer == 0){ // Send AT every two seconds and wait for the answer
answer = sendATcommand("AT", "OK", 2000);
}
}
}
int8_t sendATcommand(char* ATcommand, char* expected_answer, unsigned int timeout){
uint8_t x=0, answer=0;
char response[100];
unsigned long previous;
memset(response, '\0', 100); // Initialice the string
delay(100);
while( Serial.available() > 0) Serial.read(); // Clean the input buffer
Serial.println(ATcommand); // Send the AT command
x = 0;
previous = millis();
// this loop waits for the answer
do{
if(Serial.available() != 0){ // if there are data in the UART input buffer, reads it and checks for the asnwer
response[x] = Serial.read();
x++;
if (strstr(response, expected_answer) != NULL) // check if the desired answer is in the response of the module
{
answer = 1;
}
}
}while((answer == 0) && ((millis() - previous) < timeout)); // Waits for the asnwer with time out
return answer;
}

Answer 1

char phone_number[]="*********";
In the above code, replace ********* with 9xxxxyyyyy (Sim Number/ Mobile Number)
char phone_number[]="*********";
Then start the Serial Monitor with the mentioned baud rate (115200 as in the above code)