phreak (n.)

1972 (also as a verb), originally in phone phreak, one of a set of technically creative people who electronically hacked or defrauded telephone companies of the day.

The phreaks first appeared on the US scene in the early 1960s, when a group of MIT students were found to have conducted a late night dialling experiment on the Defense Department's secret network. They were rewarded with jobs when they explained their system to Bell investigators. ... The name "phone phreak" identified the enthisiasts with the common underground usage of freak as someone who was cool and used drugs. [New Scientist, Dec. 13, 1973]

The ph- in phone may have suggested the alteration, and this seems to be the original of the 1990s slang fad for substituting ph- for f- (as in phat).