Green’s Dictionary of Slang

(US) a person who uses special equipment to obtain free calls from the telephone system.

1973		New Scientist 5 Apr. 23: So far, British phreaks have tended to avoid the sophisticated electronic devices used by many US phreaks.
1999		Guardian 9 Oct. 🌐 Spiby went undetected for six months, during which he ran up 64 whole days on the phone and earned his place as a ‘phreaker’ – a phone system hacker.