Heat oven to 350. Spray 13x19- inch baking pan with no-stick cooking spray. Prepare cake mix as directed on package. Pour 2 cups batter into prepared pan, bake for 15 minutes.
Melt caramels and butter in heavy saucepan over low heat. Remove from heat, add sweetened condensed milk; mix well. Spread caramel mixture evenly over cake; spread remainging cake batter over caramel mixture.
Return to oven and bake and additional 25 minutes or until cake springs back when lightly touched. Cool. mix caramel topping and pecans in small bowl. Spread over cooled cake and serve.