# Placing outbound calls

The application initiates an outbound call by sending an ACU_CONN_RQ message to the protocol stack. The connection ID is assigned by the user. This ID must not currently be in use; otherwise the connection request is rejected by the ISDN protocol stack. The ACU_CONN_RQ message must contain the complete called number.

At this point, an ACU_CALL_PROC_IN event or an ACU_ALERT_IN event can be received from the network. These messages indicate that the call is in progress. If the call is successfully connected, the ACU_CONN_CO event is received. Otherwise a hang up indication, ACU_CLEAR_CO, is received.

Note: The bits in out_calls_behaviour (in the ISDN_PROTOCOL_PARMS structure passed to isdnStartProtocol) control stack behavior with outgoing calls.

The following illustration shows the sequence of messages sent between the ACU and the application for an accepted outbound call: