Under normal conditions, call clearing is usually initiated when the user or the network sends a DISCONNECT message and follows the procedures defined in 5.3.3 and 5.3.4 respectively. The only exceptions to the above rule are as follows:
a) In response to a SETUP message, the user or network can reject a call (e.g. because of the unavailability of a suitable B-channel) by responding with a RELEASE COMPLETE message provided no other response has previously been sent (e.g. the SETUP ACKNOWLEDGE message in the case of overlap sending); releasing the call reference, and enter the Null state.
b) In the case of a multipoint terminal configuration, non-selected user call clearing will be initiated with RELEASE message(s) from the network (see 5.2.9). The RELEASE message shall contain cause No. 26, non-selected user clearing.
c) Clearing of temporary signalling connections will be initiated by sending a RELEASE message as described in 5.3.3 and 5.3.4.
d) Unsuccessful termination of the B-channel selection procedure (see 5.2.3.1 and 5.1.2) by the side offering the call is accomplished by sending a RELEASE message. The RELEASE message shall contain cause No. 6, channel unacceptable. The network and user shall subsequently follow the procedures of 5.3.3 and 5.3.4.
e) 1) In the case of a SETUP message sent via the broadcast data link, if a network disconnect indication is received during call establishment, and prior to the expiry of timer T312, timer T303 is stopped(if running) and the network enters the Call Abort state. Any user which has responded, or subsequently responds before timer T312 expires, will be cleared by a RELEASE message (with the cause code(s) contained in the network disconnect indication) and the procedures of 5.3.4 are then followed for that user. Upon expiry of timer T312, the network shall treat any subsequent responses according to the procedures defined in 5.8.3.2. The network shall enter the Null state upon completion of clearing procedures for all responding users.
2) In the case of a SETUP message sent via the broadcast data link, if a network disconnect indication is received during call establishment after expiry of timer T312, any user which has responded shall be cleared by a RELEASE message (with the cause code(s) contained in the network disconnect indication) and the procedures of 5.3.4 are then followed for that user. The network enters the Null state upon completion of clearing procedures for all responding users.
Note - A separate state machine exists for each responding user.
f) When timer T318 expires, the user initiates call clearing by sending a RELEASE message with cause No. 102 recovery on timer expiry; starting timer T308, and continuing as described in 5.3.3.