Section: 5.2.1.4.1 Notification of interworking in connection with mobile originated call establishment
Content: During call establishment, the call may leave a PLMN/ISDN environment; e.g., because of interworking with another network, with a non-PLMN/ISDN user, or with non-PLMN/ISDN equipment within the called user's premises; the call may also return to a PLMN/ISDN environment. When such situations occur, the network may send a progress indicator information element to the calling mobile station either: a) in an appropriate call control message, if a state change is required (e.g. ALERTING or CONNECT); or, b) in the PROGRESS message, if no state change is appropriate. This progress indicator information element shall contain one of the following progress description values: a) #1 "call is not end-to-end PLMN/ISDN; further call progress information may be available in-band". b) #2 "destination address is non-PLMN/ISDN". c) #4 "call has returned to PLMN/ISDN. See also subclauses 5.5.1 and 5.5.6 for further reactions of the mobile station.