PREHEAT oven to 425°F. Pull cheese into thin strips; set aside. Unroll pizza crust on work surface; cut into 6 squares. Place on lightly greased baking sheet.
SPOON about 1 Tbsp. each of the ricotta cheese and pizza sauce onto each square; sprinkle evenly with string cheese. Fold each square in half; pinch edges together to seal.
BAKE 10 to 12 min. or until lightly browned. Cool slightly. Meanwhile, microwave remaining pizza sauce in microwaveable bowl on HIGH 30 sec. or until heated through. Serve with the calzones.