Callus is not the right word for it. Maybe a very thin, very superficial blister. I'm sure there's a right word for it, but it's escaping me.
The outer, thinner skin of the lip is "charred" in a small spot. Whiteish, rough to the touch, but oh so subtle. I tried to take a picture, but it won't show.
It doesn't hurt, but when I'm skmoking it tingles a bit.
I tried breath smoking, but it's not my thing. I like to take long, slow sips, play with the smoke in my mouth, and very slowly let it go. I do sip more frequently that I should, I reckon.
I'm smoking a lot of filtered cobs without the filter, and it tends to get wet sometimes. That might be also the cause.