During call establishment, the call may leave the ISDN environment, e.g. because of interworking with another network,with a non-ISDN user, or with non-ISDN equipment within the called user's premises. When such situations occur a progress indication shall be returned to the calling user either:
a) in an appropriate call control message when a state change is required: CALL PROCEEDING, ALERTING, SETUPACKNOWLEDGE 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 messages sent to the user, (for further information see Annex G).
1) No. 1 - Call is not end-to-end ISDN; further call progress information may be available in-band;
2) No. 2 - Destination address is non-ISDN;
3) 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 5.1 apply. If the Progress indicator information element is included in the Progress message, no state change will occur but any supervisory timers shall be stopped except network timer T302. In both cases,if indicated by the Progress indicator information element, the user shall connect to (if not connected already) and then monitor the B channel for further in-band information.
If the interface at which the progress indication originates is the point at which a call enters the ISDN environment from a non-ISDN environment, one or more of the following progress indicator information elements shall be included in the SETUP message sent to the network:
i) No. 1 - Call is not end-to-end ISDN; further call progress information may be available in-band;
ii) No. 3 - Origination address is non-ISDN.