People are often unsure of whether to use "who" or "whom." A quick way to find the right word is to mentally substitute "he" or "him." [Note to pedants: I know there is a split infinitive in the previous sentence, and I don't care.]
If "he" fits the sentence better, use "who." If "him" fits better, use "whom."
Who/whom is calling?
Give the letter to who/whom?
This man, who/whom I never trusted, became my boss.
He is calling. (Who is calling?)
Give the letter to him (Give the letter to whom?)
[We're going to rearrange the clause in the next sentence slightly]
I never trusted him. (This man, whom I never trusted, became my boss.