Green’s Dictionary of Slang

yell v.

(US Und.) to confess; to betray and/or testify against an accomplice; to register a complaint.

[US]Goldin et al. DAUL 242/1: Yell, v. 1. To confess one’s guilt to police; to confess and implicate one’s accomplices; to give testimony against an accomplice or any underworld associate.
[US]‘Ed Lacy’ Men from the Boys (1967) 13: You slobs made a mess of this room, ruined the rug. Be trouble if the chambermaid yells.

In compounds

yell phone (n.)

(N.Z. prison) a cell lavatory.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 205/1: yell phone n. a cell toilet.

In phrases