If a private ISDN is attached to the access at the destination interface, the following procedures are applicable at call request. The private ISDN acts as the called user.
If the calling user allows fallback to occur to an alternative bearer capability, then the network shall indicate this to the called user by means of repeated Bearer capability information elements within the SETUP message sent to indicate the presence of a call request.
The order of the information elements shall indicate the priority of the bearer capabilities. Bearer capability information elements shall be in ascending order of priority, i.e. a subsequent Bearer capability information element shall indicate a bearer capability with higher priority.
If fallback allowed is indicated in the SETUP message as described above, and fallback occurs at the destination user(beyond the private ISDN) or fallback does not occur, the user shall include in the CONNECT message sent to the network the Bearer capability information element of the resultant bearer service or teleservice.
If fallback allowed is indicated in the SETUP message as described above, and fallback occurs within the private ISDN,the user shall include in a PROGRESS message or other appropriate call control message sent to the network a Progress indicator information element with a progress description No. 5, interworking has occurred and has resulted in a telecommunication service change. The user shall include the Bearer capability information element of the resultant bearer service or teleservice.
When a PROGRESS message is sent containing a Progress indicator information element with progress description No. 5, interworking has occurred and has resulted in a telecommunication service change, neither the user nor the network shall stop timers described in 5.2.6 as a result of this action.