Cook the soba or spaghetti according to the package directions.
Meanwhile, place the broth and scallions in a pot and bring to a simmer. Add the carrots and simmer for 2 minutes.
Rinse the scallops. Add to pot and cook until they are the same color throughout, 5 to 7 minutes. Remove from heat and stir in the soy sauce.
Divide the soba or spaghetti among bowls. Ladle the soup over the noodles. Serve with additional soy sauce on the side.