The ISUP Initial Address Message – IAM – contains a number of mandatory parameters that are used to help with establishing a telephony call or a ISDN supplementary service.
There is a flag called the Forward Call Indication (FCI) that is used to help with setting up calls to ISDN and non ISDN Networks. The Interworking Indicator is used to help with interworking with non-ISDN or legacy based, e.g. analog, Networks.
Manipulation of the above flags will be done by the Switch Control/Signalling software.