Hey 
I'm new. Have connected SIM800L to my Arduino UNO R3, but just flashes green two times and a break.
Even when I do not have SIM cards in making it instantly. Also no respont on AT command!
pinout:
| ARDUINO UNO >>> SIM800L |
|---|
| GND GND >>> |
| RX 10 >>> TX |
| TX 11 >>> RX |
| RESET 2 >>> RST |
POWER SOURCE 5V (From arduino uno r3) >>> VCC