The second operating system hiding in every mobile phone
Though about GSM, if you want to learn more:* http://osmocom.org* http://www.youtube.com/watch?v=xOp_wtsHAe8* http://www.youtube.com/watch?v=_0LCgxe24Po* http://www.youtube.com/watch?v=9cBJV3yTaQo* http://www.youtube.com/watch?v=9cBJV3yTaQoYour phone has GSM, even if you're only on 3G or 4G networks though (unless it's a pure CDMA phone) - and the concepts are anyway quite similar in 3G/4G networks an phones.