Phone Activity Status



According to AT Command Interface Guide, GSM module can be in 6 different states, that can be determined by AT+CPAS command.
0 - ready
1 - unavailable
2 - unknown
3 - ringing
4 - call in progress
5 - asleep
Where can I get more detailed description of each state, when GSM module jumps from one state to another, which functionalities are available in each state?