From 1/2 container of frosting, reserve about 2 tablespoons white frosting. Tint about 1/4 cup of the frosting pink. Onto center of white-frosted half of cake, spread some of the pink frosting in square shape for message screen. Place remaining pink frosting in decorating bag with writing tip. Pipe pink frosting along edge of white-frosted cake. Arrange gum on cake for number buttons; pipe on numbers with pink frosting.