Used correctly, it could be used to exploit vulnerabilities in the CCITT Signaling System No. 5, used by international telephone switches of this era, to make free calls around the world.
C5 was designed for Inter-Continental traffic where many transmission paths were lengthy terrestrial, often submarine cable, and geostationary satellite links.
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International variants of signalling protocols, such as International ISUP and #5, in addition to the relevant national signalling protocols.