If at any time in the call set-up the connection cannot be completed, the exchange will (if applicable):
a) return an indication (in-band or out-band) to the calling party (see 2.2.4); or
b) attempt to re-route the call set-up; or
c) initiate release procedures to the preceding and/or succeeding exchange (see 2.2.1).
The initiating exchange immediately starts the release of the switched path (if established). The exchange sends a release message to the preceding and/or succeeding exchange and timers T1 and T5 are started to ensure that a release complete message is received from the preceding and/or succeeding exchange (expiration of timers T1 and T5 is covered in 2.9.6).
On receipt of a release message an intermediate exchange:
i) immediately start the release of the switched path; when the circuit is re-selectable, a release complete message is sent;
ii) at the same time as the start of the release of the switched path, a release message is sent to the preceding or succeeding exchange. Timers T1 and T5 are started to ensure that a release complete message is received from the preceding or succeeding exchange (expiration of timers T1 and T5 is covered in 2.9.6).
On receipt of a release message from the preceding or succeeding exchange, the controlling exchange starts the release of the switched path. In addition, the controlling exchange will (if applicable):
a) return an indication (in-band or out-band) to the calling party (see 2.2.4); or
b) attempt to re-route the call set-up; or
c) initiate release procedures to the preceding or succeeding exchange (see 2.2.1).
In case a) above an indication is carried in the call progress message or address complete message indicating in-band information is available along with the cause parameter. The cause value should reflect the reason of call failure in the same way as the in-band tone or announcement to be applied by the controlling exchange, (see 2.2.4).
When the controlling exchange is ready for circuit re-selection, a release complete message is sent to the preceding or succeeding exchange.
The applicability of tones and announcements is decided based on the transmission medium requirements. Tones and announcements are applicable for the following transmission medium requirements;
– speech;
– 3.1 kHz audio; and
– 64 kbit/s unrestricted preferred.
If a call set-up fails and no in-band tone or announcement has to be returned to the calling party from an exchange succeeding the controlling exchange, this exchange sends a release message to the controlling exchange. The cause value should reflect the reason of the call failure in the same way as the in-band tone or announcement to be applied by the controlling exchange.
If a call set-up fails and an in-band tone or announcement has to be returned to the calling party from an exchange or called party, the exchange or user concerned connects the in-band tone or announcement to the transmission path. If a time-out occurs at the exchange providing the in-band tone or announcement, the exchange sends a release message to the preceding exchange with cause value #31 (normal unspecified).
If an address complete message has been returned to the preceding exchange a call progress message indicating that in-band tone information is available along with the cause parameter, is returned to the preceding exchange (see 2.1.5). The cause value should reflect the reason of call failure in the same way as the in-band tone or announcement to be applied.
If an address complete message has not been returned to the preceding exchange already, an address complete message, with the cause parameter and the "in-band information" indicator set in the optional backward call indicator, will be returned to the originating exchange. The cause value should reflect the reason of call failure in the same way as the in-band tone or announcement to be applied.
In case a special tone or announcement has to be applied due to an event only known by a certain exchange and not covered by a cause value, no cause parameter is included in either the address complete or call progress messages. The answer message must not be sent in this case.
For the preceding exchanges the inclusion of the cause parameter in the address complete or call progress messages implies an unsuccessful call set-up. The cause parameter will not be included for unsuccessful call set-up's when interworking has occurred and the in-band tone or announcement is returned from the exchange beyond the interworking point.
The determination that the proper number of digits has not been received can be made at once if the end of pulsing signal is received.
When overlap working is used and the end of pulsing has not been received, the release message with cause 28 (address incomplete) will be sent 15-20 seconds (T35) after receipt of the latest address message and before receipt of the minimum or fixed number of digits for forward routeing of the call.