I had a waxing injury on my lower lip that eventually turned into a callus forming on my lip. The fact that I kept picking and trying to exfoliate as vigorously as I can made matters worse. It grow to about a 5 mm callus that stayed for almost 2 years now. My sister said that onions can cure callus on the feet so maybe it would help with the lip as well. so I tried and it worked in 1 week it shrunk to 2mm and is healing beautifully.
Here is the remedy.
Peel an onion. Get a piece of the transparent membrane large enough to cover the whole callus and little bit of the surrounding skin. put the wet sticky side on the callous and make sure there is no air pockets.
It helps if you can prepare the area in advance. No harsh exfoliation. Warm water with little bit oil on a cotton ball to soften the skin. If on feet you can just soak in the water. Gently rub baking soda but be very gentle with lips. Feet may handle being little bit more vigorous. Wash and pat dry, then put on the onion membrane and leave it on as long as you can. Try whole night or if you are staying home you can just walk around with it. Replace the membrane as needed. It will dry out eventually. When not using the membrane moisturize generously and don't pick at it and prevent friction as much as possible.
Here is my theory of how this works. Since callus forms as a defense mechanism of the skin to protect itself from damage or friction harsh exfoliation or scraping the area will never work. It only makes situation worse. Covering the area with thin skin like membrane ensures the body that it no longer needs protection and that the skin is safe. I am sure chemical compound of the onion also plays a healing role but I don't have any expertise on that to explain how. In short once the skin cells are convinced that they don't need to be in the defensive mode they let the new tissue grow and let go of the callus tissue.
Well this is what I have observed in healing the callus on my lip. Now I will try this on little callus on my feet about 7mm. Will try and update with results.