The woman faces five felony counts including harassment. A criminal complaint said she told police she was "bored" and "wanted to have some fun."
The woman and a 20-year-old friend, also facing charges, allegedly called the older woman on Feb. 5 and said "I'm gonna kill you," "You're going to die" and "I'm watching you."
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Police officers answered some calls for the grandmother and heard a female caller make threats.
The criminal complaint said the suspect told investigators she wanted to scare her grandmother but didn't want her dead. She said she knew it was wrong but not illegal.