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Nightmare Fuel: The Baby-Sitters Club
Claudia sits for a family in which the husband abuses the kids. After Claudia and her mother rescue the children and their mother, Claudia gets a phone call late at night. It's the abusive father, wailing at her to give him back his children.
In one of the early books, a serial thief was breaking into houses in nearby towns, making calls to prospective targets and instantly hanging up if anyone answered. The club starts getting calls where the caller hangs up once they answer, and later in the book, Claudia's babysitting and realizes that there's a prowler outside. It's one of the guys from her class, being a moron, and the calls were from another guy who couldn't work up the courage to ask her out, but it's a pretty Adult Fear-ridden situation.