Look, in the original sentence the pronoun used is 'Jou', which is the Object Pronoun for the marked (stressed) second-person singular.So you must be the object of the sentence.Therefore "Is she the one who calls you?" is the translation.If we chose to use 'Jij', which is the marked (stressed) Subject Pronoun for the second-person singular, then the sentence would be translated to "Is she the one whom you are calling?". But note that 'Je' is both the unmarked Subject Pronoun and the unmarked Object Pronoun for the second-person singular, so you cannot tell if "Is zij diegene die je belt?" means "Is she the one who calls you" or "Is she the one whom you call".
Take a look at these: Subject Pronouns: http://www.dutchgrammar.com/en/index.php?n=Pronouns.Ps02 Object Pronouns: http://www.dutchgrammar.com/en/?n=Pronouns.Ps05
That's not right at all. "wie" is not the object form of "die". From reading a previous discussion I believe that "wie" is used as a relative pronoun only when the person it refers to is unknown, e.g. "Do you know who called?". When the person is known then Dutch always uses "die".
wie is used in questions (both direct and indirect - 'do you know who called?' is an indirect question, for example, its direct version being 'who called?'); while die is a relative pronoun and is used to introduce a subordinate clause. In 'she is the one who called you', we have 'she is the one' as a main clause (well, technically the whole thing would be the main clause, I think, but let's leave it at that), and 'who called you' is a subordinate clause introduced by the relative pronoun 'who', which is acting as the Subject of the subordinate clause.
Hope this helps :)
- Who should be used to refer to the subject of a sentence.
As in the sentence: "Is she the one who calls you?" (so to answer your question - it's wrong using whom in lieu of who in this case)
- Whom should be used to refer to the object of a verb or preposition.
As in the sentence "Is she the one whom you call" (the verb here is "to call", and the object of this verb, namely, the one receiving the action is "she" and that's why whom should be used in this case).
When in doubt, try this simple trick: If you can replace the word with “he”' or “'she,” use who. If you can replace it with “him” or “her,” use whom.
So using our sentences from above:
"Is she the one who calls you?"
she\he calls you -> who
"Is she the one whom you call"
her\him you call -> whom
Duo has succeeded (again) in introducing a sentence that is far too difficult at this stage of the course! Those who thought of this sentence obviously did not realise this. But take it from me: it would have been much better if they had limited themselves to only "degene die" instead of "diegene die".
Btw, we Dutch probably say in 99% of the cases "degene die"; not: "diegene die". I've argued here before: keep it simple, and be clear! (Please: yes, I would almost forget to add that...).