Combine water and soup bone in a large stock pot; bring to a boil. Reduce heat; add tomatoes, onion, lentils, carrots, celery, salt, and pepper, stirring well. Simmer 1 1/2 hours, stirring frequently. Remove from heat; remove and discard soup bone.
Stir in frankfurters; simmer an additional 30 minutes, stirring frequently. Serve immediately in warm soup bowls.