Most professors are PhDs - and but not all PhDs are professors.

A junior teacher at a university with a doctorate could be called "Doctor so and so", but when they become a professor they call themselves that instead.

In Italy, with a Master's degree, they'll call you "Dottore" as a mark of respect, though you haven't a doctorate.

In Germany, if you have a doctorate you are Herr Doktor - but your wife also will be called Frau Doktor - not a bad deal at all.

I knew a fellow with seven degrees, including two doctorates. But his proudest achievement was to have been a combat fighter pilot, and live to tell the tale.