Form and rules
How it works
In a subject question, who/what is the subject: Who called you? Do not add did when asking who performed the action.
In an object question, use normal question structure: Who did you call?
Compare Who broke the window? with What did he break?
In everyday English, a preposition often stays at the end: Who are you talking to?
Formal English can move the preposition before whom/which, but this is not necessary for normal A2 conversation.