Possible calluses

What interesting photos!  IN particular, it appears to me (and you, being able to see this in context, may confirm or disagree) that the bald areas are NOT confined to the weight-bearing portions of the foot.  I therefore wonder whether they are caused by some kind of fungal infection, or by demodex, which is a little mite that lives in the hair-follicles and thereby tends to make hair fall out.  These could both be identified by your vet, using tests that they should have available.  What is interesting about Ringworm and Demodex, is that both are more likely to occur when animals are immunocompromised, so they can be a SECONDARY sign - a reflection that something elsewhere in the body is not going right.  Thus 'parasitic' hair-loss is seen as a result of endocrine disorders, of cancers, of certain drugs and so on.  Cancer and endocrine disease can also cause hair-loss in their own right.  If the area is painful and being licked, this could also account for widespread hair damage e.g. joint pain or skin pain (secondary, say, to dermatitis).  We would strongly recommend showing your dog to a vet, who will help to establish the cause in this case.  Once it is known, your vet can make you aware of the options to treat or control this moving forward.   Because Ringworm is on my list of differentials, please avoid touching the area with your bare hands until it has been ruled out, as people can catch this too.

Calluses are areas of hardened skin that occur as a result of friction against other services.  Unless the hairless areas are all dragging on the floor or some other hard surface, I feel that the explanation may be more complex than that.