# Receiving inbound calls

When a call SETUP message is detected on the trunk, an ACU_CONN_IN is sent to the application from sender ENT_CC. Receipt of ACU_CONN_IN triggers a state transition from the NULL state to the WAIT_INCOMING state.

The application decodes the message and obtains the calling number and called number. The application then decides to accept or reject the call based on the value of the called number.

## Accepting the call

To accept the call, the application builds a connect response message (ACU_CONN_RS) and sends it to the ISDN protocol stack. The event ACU_CONN_CO confirms that the call has been established. The stack switches to its ACTIVE state, where the application processing takes place.

Note: Applications with DSP resources that are not connected to a B channel connect calls when they receive connection indications.

The following illustration shows the default sequence of messages sent between the ACU and the application for an accepted inbound call. The actual messages sent back and forth in response to an incoming call may differ from this example depending upon the settings of the bits in the in_calls_behaviour substructure.

This substructure is referenced in the ISDN_PROTOCOL_PARMS structure passed to isdnStartProtocol. For details, see ISDN_PROTOCOL_PARMS_Q931CC parameters and ISDN_PROTOCOL_PARMS_LAPD parameters.

## Incoming calls with overlap receiving mode enabled

If overlap receiving mode is enabled, when a call arrives, the ACU sends an ACU_CONN_IN message to the application even if the called number, the calling number, or both are not complete. The ACU then sends any additional incoming digits in ACU_DIGIT_IN messages.

To enable this mode, set the CC_TRANSPARENT_OVERLAP_RCV bit in in_calls_behaviour. (This substructure is referenced in the ISDN_PROTOCOL_PARMS structure passed to isdnStartProtocol.) For details, see ISDN_PROTOCOL_PARMS_Q931CC parameters and ISDN_PROTOCOL_PARMS_LAPD parameters.

## Rejecting the call

To reject the call, the application builds a clear request message (ACU_CLEAR_RQ) and sends it to the protocol stack. When the call is released, the network responds with ACU_CLEAR_CO.

The following illustration shows the default sequence of messages sent between the ACU and the application for a rejected inbound call. The actual messages may differ depending upon the settings of the bits in in_calls_behaviour.

This substructure is referenced in the ISDN_PROTOCOL_PARMS structure passed to isdnStartProtocol. For details, see ISDN_PROTOCOL_PARMS_Q931CC parameters and ISDN_PROTOCOL_PARMS_LAPD parameters.