Baby-Sitters Club: Claudia and the phantom phone calls. Ann Martin. 2010.

Book 3 in the series 

Kristy, Claudia, Mary Anne and Stacey have had some strange adventures since they started the Baby-sitter’s Club. But nothing has 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 it 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.