Forward/Backward Call Indicators and the Progress Indicator (PI)
of ISUP messages are used in call control signals, such as for setting up a mobile
multimedia call, to indicate the involvement of a Public Switched Telephone Network
(PSTN) terminal. Either terminal of an end-to-end connection can initiate the call
and likewise either can respond to the call control signals initiated by the other
terminal to immediately recognize the unambiguous fact that a PSTN terminal is
involved in the call and to immediately respond by correctly setting up an appropriate
and compatible communication format.