Wrong Number, The (Fear Street #5) by R.L. Stine - Vintage Paperback

It begins as an innocent prank: Deena Martinson and her best friend, Jade Smith, make sexy phone calls to the boys from school. But Deena’s half-brother, Chuck, catches them in the act and threatens to tell their parents—unless the girls let him in on the fun. Chuck begins making random calls, threatening anyone who answers. It’s dangerous and exciting. The teens are even enjoying the publicity and the uproar they’ve caused.

Until Chuck calls a number on Fear Street.