These IEs may be used with most Layer three Call Establishment messages and with the Disconnect message in the clearing sequence. They cater for interworking configurations between ISDN and non ISDN equipment and/or networks.

They are used by the internetworking function to inform the ISDN TE that the TE is establishing a call with a non ISDN network or device. They are used similarly to inform the TE of the origin of incoming calls from non ISDN networks and/or equipments.

A demonstration of how a Progress Indicator is used is illustrated in Low Layer Compatibility IE and BCC IE Layer Compatibility Issues, Note 1.