Calluses is a thickening of the outer layer of skin. This thickening is known medically as hyperkeratosis. Calluses develop as part of the skin's normal defense against prolonged rubbing, pressure and other forms of local irritation.

Solution:
Always use a pumice stone.
Razors and other cutters are now illegal for pedicurists to use. Avoid wearing high-heel shoes. For more on Calluses Management, click here