When the user determines that sufficient call set-up information has been received and compatibility requirements (see Annex B) have been satisfied, the user responds with either a CALL PROCEEDING, ALERTING, or CONNECT message (see Note), and enters the Incoming Call Proceeding, Call Received, or Connect Request state respectively.
Note - A Progress indicator information element may be included in CALL PROCEEDING, ALERTING and CONNECT messages (e.g. when an analogue terminal is connected to an ISDN PABX). The CALL PROCEEDING message may be sent by the user which cannot respond to a SETUP message with an ALERTING, CONNECT, or RELEASE COMPLETE message before expiration of timer T303.
When the SETUP message was delivered via a broadcast data link, an incompatible user shall either
a) ignore the incoming call; or
b) respond by sending a RELEASE COMPLETE message with cause No. 88, incompatible destination, and enter the Null state. The network processes this RELEASE COMPLETE message in accordance with 5.2.5.3.
When the SETUP message was delivered via point-to-point data link, an incompatible user shall respond with a RELEASE COMPLETE message with cause No. 88, incompatible destination. The network processes this RELEASE COMPLETE message in accordance with 5.2.5.3.
A busy user which satisfies the compatibility requirements indicated in the SETUP message shall normally respond with a RELEASE COMPLETE message with cause No. 17, user busy.The network processes this RELEASE COMPLETE message in accordance with 5.2.5.3.
If the user wishes to refuse the call, a RELEASE COMPLETE message shall be sent with the cause No. 21, call rejected,and the user returns to the Null state. The network processes this RELEASECOMPLETE message in accordance with 5.2.5.3.