Claudia and the Phantom Phone Calls by Ann M. Martin
Being a good babysitter isn't always easy, and the vice-president of the Babysitters Club, Claudia Kishi, is learning that the hard way. She and the other club members have started getting mysterious calls on the job. Is it the Phantom Caller, a jewel thief who's been breaking into houses in the area? One thing is certain - the friends need to take action to protect the kids in their care!