Green’s Dictionary of Slang

(Aus./US prison/Und.) a prisoner or criminal who is overly friendly towards the authorities.

1954		Sun. Mail (Adelaide) 25 Sept. 46/2: ‘The Thing’ [i.e a criminal] was a Walkie-Talkie.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Walkie talkie. An epithet laid on a person who talks excessively.
1992		Bentley & Corbett Prison Sl. 37: Walkie-Talkie An inmate who is frequently seen talking to guards or staff.