// send an SMS!
    char sendto[21], message[141];
    flushSerial();
    Serial.print(F("Send to #"));
    readline(sendto, 20);
    Serial.println(sendto);
    Serial.print(F("Type out one-line message (140 char): "));
    readline(message, 140);
    Serial.println(message);
    if (!fona.sendSMS(sendto, message)) {
      Serial.println(F("Failed"));
    } else {
      Serial.println(F("Sent!"));
    }

char sendto[21], message[141];

void loop()
{
  //This is for enabling sms sending procedure

  boolean sendSMS = false;

  //This is for storing measured sensor value
  int analogValue0 = analogRead(A0);
  int upperLimit = 650, lowerLimit = 300;

  if(analogValue0 > upperLimit && sendSMS == true)
  {
    sendSMS = false;

    // send an SMS!
    // Here you'll send an sms.
    //...
  }

  if(analogValue0 < lowerLimit && sendSMS == false)
  {
    sendSMS = true;
  }
}