as an expansion on a previously posted deffinition:

A phreak is anyone who explores phone systems. Basically a phone hacker

Phreak (or phreaking/phreaker) is a term used to describe the early hackers in the late 70's-early 90's who manipulated the tone-based telephone switching systems to conduct unauthorized activity within that system. Although these "phreakers" rarely had malicious intent, their curiosity often got them arrested and heavily punished.

Someone who likes to explore the workings of the telephone system, such as switching, call routing, etc. Phreaks are always trying to keep up with the latest telecom innovations.

A term of exclamation used to describe a person, in reaction to something silly and/or humourous they say.

Not necessarily used as an insult, this term is most commonly used as a term of endearment and should never be taken seriously.

Taken from a combination of the words freak and prick.

A loyal fan of the jam band Umphrey's McGee. (See Umphreak).

(noun) A person obsessed with the illicit application of fringe technology.

In order to phreak you must have an advanced knowledge of the:
Telephone System
Landlines
Cell Phones
and the operations the operator can conect

Also you need to know how to create/do (a):
tone
open up a payphone
cross the correct wires in a payphone
and know the operations of the area codes