user30000: I have a SIM808 module like this: https://www.banggood.com/SIM808-Module-GPS-GSM-GPRS-Quad-band-Development-Board-For-Arduino-p-1063106.html
The output header has 5 pins: GND/RX/GND/TX/VCC and I connected them as: 
GND/VCC to a LiPo battery, 7.4V@1000mAh
RX->pin 14 (written on it TX3)
GND->to arduino GND
TX->pin 15 (written on it RX3)
and uploaded this code
#include <SoftwareSerial.h>


SoftwareSerial GSMSerial(15,14);    // RX, TX

void setup()
{

  GSMSerial.begin(19200);
  Serial.begin(19200);
  Serial.println("START");
}

void loop()
{
  if(Serial.available())
    GSMSerial.print((char)Serial.read());
  else  if(GSMSerial.available())
    Serial.print((char)GSMSerial.read());
}

But I'm not getting anything on it (when sending "AT" or anything else). 
Any help is appreciated. 
Thank you!

user14737: Don't use SoftwareSerial on the HardwareSerial pins.  Instead of this:
#include <SoftwareSerial.h>
SoftwareSerial GSMSerial(15,14);    // RX, TX

Just do this:
#define GSMSerial Serial3


30000: <p>I have a SIM808 module like this: <a href="https://www.banggood.com/SIM808-Module-GPS-GSM-GPRS-Quad-band-Development-Board-For-Arduino-p-1063106.html" rel="nofollow noreferrer">https://www.banggood.com/SIM808-Module-GPS-GSM-GPRS-Quad-band-Development-Board-For-Arduino-p-1063106.html</a></p>

<p>The output header has 5 pins: GND/RX/GND/TX/VCC and I connected them as: </p>

<p>GND/VCC to a LiPo battery, 7.4V@1000mAh</p>

<p>RX->pin 14 (written on it TX3)</p>

<p>GND->to arduino GND</p>

<p>TX->pin 15 (written on it RX3)</p>

<p>and uploaded this code</p>

<pre><code>#include &lt;SoftwareSerial.h&gt;


SoftwareSerial GSMSerial(15,14);    // RX, TX

void setup()
{

  GSMSerial.begin(19200);
  Serial.begin(19200);
  Serial.println("START");
}

void loop()
{
  if(Serial.available())
    GSMSerial.print((char)Serial.read());
  else  if(GSMSerial.available())
    Serial.print((char)GSMSerial.read());
}
</code></pre>

<p>But I'm not getting anything on it (when sending "AT" or anything else). </p>

<p>Any help is appreciated. </p>

<p>Thank you!</p>
 14737: <p>Don't use <code>SoftwareSerial</code> on the <code>HardwareSerial</code> pins.  Instead of this:</p>

<pre><code>#include &lt;SoftwareSerial.h&gt;
SoftwareSerial GSMSerial(15,14);    // RX, TX
</code></pre>

<p>Just do this:</p>

<pre><code>#define GSMSerial Serial3
</code></pre>
