"What is her phone number?"
Not a native speaker, but I think usually the subject or object of the sentence should be established first (in this case, the girl's phone number), before taking an action on it (such as asking for it).
The transliteration of the Japanese sentence would be something like, "As for her phone number, what is it?" It doesn't flow too well in English (hence the official answer is "What is her phone number?"), but that's one of the idiosyncrasies of the language I reckon.
の no - is a particle that denotes possession. は ha (voiced wa) - is a particle that indicates the topic of the sentence.
かのじょの電話ばんごうは何ですか kanojo no denwa bangou wa nani desu ka
かのじょの電話ばんごう, that means “her phone number” is the topic of the sentence and that's why は goes after it.