Who vs. whom: does anyone still care?

The two-second test: rephrase the sentence and answer with he or him. If he fits, use who; if him fits, use whom. Who called? He called. Whom did you call? I called him. The m's even match. Grammatically, who is a subject, the one doing the action, and whom is an object, the one receiving it, which is also why whom always follows prepositions: to whom, for whom, with whom.

Now the honest sociolinguistics: whom is in centuries-long decline, everyday speech has mostly abandoned it, and even careful writers accept who at the start of questions, who did you ask, as standard informal English. Meanwhile the hypercorrect whom, deployed wrongly to sound elevated, whom shall I say is calling when the answer is he, reads worse than never using it at all.

The equilibrium: apply the him test in formal writing, use whom after prepositions, and otherwise relax. Language is leaving whom behind gently, and you are allowed to walk at its pace.