Who or whom?

They could be rewritten as :

'To whom does that book belong ?'

'To whom is the letter addressed ?

'To whom did I call ?

'But a native speaker corrected all the "who" to "whom" in each sentence' :  that would be incorrect, and a native speaker should know better.

The versions using 'whom' would not normally be used in speaking, and even in writing are rather formal.  Few people these days would use this form unless wishing to appear formal, so it might be used in an ironic [slightly sarcastic] fashion  i.e. making fun of a situation.

Who does that book belong to?

don't worry everyone says the above in real life