Green’s Dictionary of Slang

(US) to use a variety of special equipment (‘black boxes’, ‘blue boxes’) to obtain free calls from the telephone system.

2000

E. Raymond Jargon File Version 4.2.3 🌐 phreaking [...] 1. The art and science of cracking the phone network (so as, for example, to make free long-distance calls). 2. By extension, security-cracking in any other context (especially, but not exclusively, on communications networks) (see cracking). [...] A few old-time hackers still phreak casually just to keep their hand in, but most these days have hardly even heard of ‘blue boxes’ or any of the other paraphernalia of the great phreaks of yore.