It happens that Tim+ formulated :
Just get yourself a "Ped-egg". It's like a small parmesan grater for your
feet.

Much easier if your feet have had a soak first.

I don't have that much of a problem, but I like to take care of my feet
so bought one of those. I found it a bit too course and so sharp it
would dig itself into the none hard skin. Quite painful.

I found works well is a device like the Schole (sp?) type unit
advertised on TV, abrasive on a battery powered roller. Except they are
not that powerful, press too hard and they can be stopped. Even that
was capaple of generating enough heat to be painful unless it was kept
moving. They work best when the skin is very dry.