Try this.
- Set Jumper in SIM900 module to work with hardware Serial, not de pins 7&8, you will use 0&1.
- Next, connect SIM900 module to Arduino connecting:
SIM900 Arduino
pin 0 -----------> pin 1
pin 1 -----------> pin 0
GND -----------> GND
5v -----------> 5v
GND -----------> GND
- Connect Arduino board to PC using USB port.
- Empty the Arduino board, dont load anything.
- Open Monitor and use 9600 or 19200 baud.
- Type AT and press ENTER. You will receive an OK or 0 message.
If you receive a response, then you have connection and your modem is working fine.
Next steps are:
- Check the SIM card, area able to call and send SMS. You can check it using a smartphone.
- Put the SIM card in your SIM900 board and check it is connected to net with command AT=CSQ?, you will receive a +CSQ : 0.XX.
Good luck