The Gossip War — Francine Pascal

Jessica Wakefield and her twin sister, Elizabeth, have persuaded their parents to get three-way calling for their phone.  Jessica is psyched--more gossiping, more matchmaking, more fun!  There's just one problem.  Jessica didn't read the directions carefully, and when she thinks she's dishing gossip to Lisa Fowler, she's also dishing it to Ellen Riteman.  And the gossip is about Ellen!  The gossip spreads until the entire Unicorn Club in an all-out gossip war!  Can Jessica turn her telephone tricks around and win her friends back?