The Baby-Sitters Club: #2 Claudia and the Phantom Phone Calls - By Ann M. Martin

Kristy, Claudia, Mary Anne and Stacey have had some strange adventures since they started the Baby-sitters Club. But nothing's been as spooky as what's going on right now.The baby-sitters have been getting mysterious phone calls when they're out on their jobs. When a phone rings and they pick up, there's no one on the other end of the line. Claudia's sure it's the Phantom Caller, a jewel thief who's been operating in the area. Claudia has always liked reading mysteries, but she doesn't like it when they happen to her. So she and the baby-sitters decide to take action-with some very mixed results!