The Baby-Sitters Club Claudia and the Phantom Phone Calls Book 2 by Ann M. Martin

 Kristy, Claudia, Mary Anne, and Stacey try to be prepared for anything when they baby-sit. So when they hear about the Phantom Caller, a jewel thief who's been breaking into nearby homes, they come up with a plan to keep their kids safe.But when Claudia and the other girls start receiving creepy phone calls while they're out on jobs, they start to get really spooked. Will the mystery caller scare off the BSC?