The Grim Adventures of Billy and Mandy (2001)

S5E10 Prank Call of Cthulu

When Billy and Irwin embark on a series of prank calls, it's clear to everyone that Billy is the mastermind, as caller ID reveals his identity. While rummaging through Grim's basement for a way to bypass caller ID, Billy discovers a mysterious skull-shaped phone that lacks any caller information. Grim cautions him against using it, revealing that it’s a direct line to the ancient demon Cthulhu, whose mere presence drives people insane and threatens global devastation. Ignoring Grim's warning, Billy and Irwin decide to make a call, only to be ensnared by ominous tentacles.