Reset Pin

Hi all, any of you tried to use the RST pin of the GSM Shield?
I read (and tried) that if you send 0V (LOW) to a RST pin on Arduino, it will reboot. I tried it with other MEGA board, within a proper pin and it seems to work, something like this:

int led = 13;//pin 13 as OUTPUT LED pin
int resetPin = 12;
// the setup routine runs once when you press reset:
void setup() {  
  digitalWrite(resetPin, HIGH);
  delay(200);
  // initialize the digital pin as an output.
  pinMode(led, OUTPUT);    
  pinMode(resetPin, OUTPUT);     
  Serial.begin(9600);//initialize Serial Port
  Serial.println("reset");//print reset to know the program has been reset and 
  //the setup function happened
  delay(200);
}

// the loop routine runs over and over again forever:
void loop() {
  delay(10);
  digitalWrite(led, HIGH);   // turn the LED on (HIGH is the voltage level)
  Serial.println("on");
  delay(1000);               // wait for a second
  digitalWrite(led, LOW);    // turn the LED off by making the voltage LOW
  Serial.println("off");
  delay(1000);               // wait for a second
  Serial.println("resetting");
  delay(10000);
  digitalWrite(resetPin, LOW);
  Serial.println("this never happens");
  //this never happens because Arduino resets

}

But trying the same action with the MEGA attached to the GSM shield, nothing happens, and its weird, because the RST pin of the shield is attached directly to the RST pin in the mega, maybe it needs other type of voltage to to the reset thing?
I really need to restart my Sketch for a moment every X days to avoid the hanging issue, and this seems to be the cleanest way; that watchdog timer and sleep mode don´t seem to be easy to work.

Any ideas? Thank you