Sometime, the message mentioning the CallState can be seen in log. No official documentation about the states, following table is based on observations.
Messages from "CallControl" subsystem looks like:
x is the call appearance index and is useful for when development engineering is tracing the call object
M is new call state, N is previous call state
Recognized Call States
Call, originating phone: 0->6(->7)->8->9->13->(parties talking)->11->0
Call, answering phone: 0(->7)(->??)->10->12->13->(parties talking)->11->0