I am trying to interface the SIM900 shield with the Arduino UNO. I am using the code from http://tronixstuff.com/2014/01/08/tutorial-arduino-and-sim900-gsm-modules/ to try and send a message from the shield to my phone. I am using an external power supply, an o2 SIM ( i am based in the UK) however i see nothing on the serial monitor/or serial tools when I send AT and i am not receiving a message.