Warts. Calluses are usually due to excessive pressure against a particular bone. Calluses on top of the big toes may be due to the toes rubbing against the shoes or they may be warts. If they are "strange" and have little black dots in them, then they are probably warts. You can get them evaluated and treated by a podiatrist.
Depends on the cause. Many things can cause calluses. Some reasons that one may develop a callus include: viral infections (warts), friction, pressure, repedetive trauma, and a limitation of joint motion. Some genetic conditions may also cause people to develop calluses on the hands and the feet. Treatment depends on the cause of the problem. Have it evaluated by a professional and get the appropriate treatment.