Why are they not found on touch tone phone sets?
These are extensions to the standard touch-tones (0-9, *, #) which originated with the U.S. military’s Autovon phone network. The original names of these keys were FO (Flash Override), F (Flash), I (Immediate), and P (Priority) which represented priority levels that could establish a phone connection with varying degrees of immediacy, killing other conversations on the network if necessary with FO being the greatest priority, down to P being of lesser priority. The tones are more commonly referred to as the A, B, C and D tones respectively, and all use a 1633 Hz as their high tone. Nowadays, these keys/tones are mainly used in special applications such as amateur radio repeaters for their signalling/control. Modems and touch tone circuits tend to include the A, B, C and D tones as well. These tones have not been used for general public service, and it would take years before these tones could be used in such things as customer information lines; such services would have to be compatibl
