During call establishment the call may enter an ISDN environment, e.g. because of interworking with another network,with a non-ISDN user, or with non-ISDN equipment within the calling or called user's premises. When this occurs, the point at which the call enters an ISDN environment shall cause a Progress indicator information element to be included in the SETUP message to be sent to the called user:
a) No. 1 "call is not end-to-end ISDN; further call progress information may be available in-band";
Note - On receipt of progress indicator No. 1, the called user shall connect to the B-channel in accordance with the procedures of 5.2.8.
b) No. 3 "origination address is non-ISDN".
In addition, the user shall notify thecalling party if the call has left the ISDN environment within the called user's' premises, or upon the availability of in-band information/patterns.When such situations occur, a progress indication shall be sent by the user to the network either:
a) in an appropriate call control message when a state change is required (SETUP ACKNOWLEDGE, CALL PROCEEDING,ALERTING, or CONNECT); or
b) in the PROGRESS message when no state change is appropriate.
One of the following progress description values shall be included in the Progress indicator information element in the message sent to the network (for further information, see Annex G):
i) No. 1 - Call is not end-to-end ISDN; further call progress information may be available in-band;
ii) No. 2 - Destination address is non-ISDN;
iii) No. 4 - Call has returned to the ISDN.
If the progress indicator information element is included in a call control message, the procedures as described in the rest of 5.2 apply. If the progress indicator information element is included in the PROGRESS message, no state change will occur and all supervisory timers running shall be continued.
NOTE – If progress description No. 8 is received, it has no impact on any supervisory timers and shall be ignored by the network except when the Annex K procedures are applied.