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ISDN Software Reference for Linux and Windows
16
Table 2. Call Control States
Call State Description
Accepted
An indication to the network that the incoming call has
been accepted, but has not been connected to the end
user. (Most voice applications do not need this.)
Alerting
The destination is reached and the phone is ringing. This
state may be reported to the application or masked
depending on the application directive.
Connected
An incoming or outgoing call is established. Typically,
billing begins at this point and the B channel is cut
through.
Dialing
Address and call setup information has been sent to, and
acknowledged by, the network. Call establishment is in
progress.
Disconnected
The network terminates the call and the application
should drop the call.
Idle
A call is dropped and waiting for the application to
release the call reference number (CRN).
Null
No call is assigned to the device (time slot or line).
Offered
An incoming call is offered by the network.
The call states change according to the sequence of functions called by the
application and the events that originate in the network and system hardware. The
current state of a call can be changed by:
• function call returns
• termination events (indicate completion of a function)
• unsolicited events
The way calls transition between states differs depending on whether the
application uses asynchronous or synchronous programming: