5 (SS5) is a multi-frequency (MF) telephone signaling system that was in use from the 1970s for International Direct Distance Dialing (IDDD).
[1] It was also nicknamed Atlantic Code because it was used for the first IDDD connections between Europe and North America.
SS5 was designed for inter-continental traffic for which many transmission paths were long terrestrial, often submarine cable, and geostationary satellite links.
This Line signalling element was 2VF to introduce the possibility of more meanings and tone-off idle in order that hundreds of channels in transmission media would not be transmitting standing tones simultaneously.
The deficiencies of SS5 are widely known as the root cause for "Blue box fraud" that enabled phone phreaks, such as Captain Crunch (John Draper), to abuse the telephone system and place telephone calls for free, among exploiting other capabilities.