SIM800l is not Avalible I am using :
-arduino uno r3
-SIM800L

connection:

Arduino   SIM800L
5v     -->  vcc 
gnd    -->  gnd
pin10  --> TX
pin11  --> RX

the led on the module blinks once each 3~5 seconds
but when i use the function sim.available() i got 0; 
thus as i guess there is a problem .

the code :

#include &lt;SoftwareSerial.h&gt;
SoftwareSerial sim(10, 11);
int _timeout;
String _buffer;
String number = "000000000000"; //-&gt; change with your number
void setup() {
  delay(7000); //delay for 7 seconds to make sure the modules get the signal
  Serial.begin(9600);
  _buffer.reserve(50);
  Serial.println("Sistem Started...");
  sim.begin(9600);
  delay(1000);
  Serial.println("Type s to send an SMS, r to receive an SMS, and c to make a call");
}
void loop() {
  Serial.print(Serial.available());
  if (Serial.available() &gt; 0)
    switch (Serial.read())
    {
      case 's':
        SendMessage();
        break;
      case 'r':
        RecieveMessage();
        break;
      case 'c':
        callNumber();
        break;
    }
  if (sim.available() &gt; 0)
    Serial.write(sim.read());
}


i looked the internet they say the voltage should be 3.7~4.2 itriedgivving 3.55 and it didn't work
Try to connect your module with the power supply useing wires as short as are is possible. I was fight with this module for few months and now I know that almost all of issue with it are regarding power supply.
For short test you can connect module directly to LiPo 3,7V.
You have to be sure that your power supplay can handle 2A current. The voltage never can drop below 3,4V.

The power IC and the bypass capacitor should be placed to the module
as close as possible.

SimCom, SIM800_Hardware Design_V1.09, available 27.11.2020