Signaling System No. 5

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CCITT5 was a multi-frequency (MF) telephone signalling system in use in the 1970s for International Direct Distance Dialing (IDDD). It was sometimes nicknamed "Atlantic Code" because the first IDDD connections between Europe and North America used it. Based on the Bell System MF signalling system known by CCITT as Regional System R1, it had six frequencies: 700, 900, 1100, 1300, 1500 and 1700 Hz. The first five frequencies were used in a two out of five code to represent decimal numbers (phone numbers), and the last frequency in combination with one of the others represented the beginning or end of a sequence of digits.

CCITT5 differed from the CCITT4 system that was used among European countries, which was based on compelled interregister R2 signalling. It differed from the R1 multi-frequency system on which is was based, in allowing more digits to be sent in a phone number, at a slightly higher rate.