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
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...).