"What is your phone number?"
No, that translates to "what number?"
When using 何 you usually put it in the place of the info youl want in the sentence.
Ex: ●鳥は「緑」です。 (the bird is green) ○鳥は「何色」ですか？ (What color is the bird)
And since the full answer to our question is going to be: 電話番号は*です。
We are going to replace the number (The wanted info) with 何
you can use 電番 as a short cut to 電話番号
Sometimes, a literal translation can be more helpful.
Phone number is what?
Phone number is what number? (This is more preferred way of asking in Japan and Korea.)
What's the number? (Used when both parties understand that they're talking about phone numbers.)
お is for politeness, but it also makes clear distinction between personal information and other's information: here, it is their phone number, not yours. Similarly, when speaking to people, you will say their family members with お for almost any member; for your own or when referencing the members of others, you will use the personal words.
In some cases, the お is just commonplace to use for politeness, with a nuance of the other person in consideration: お箸 (おはし), chopsticks, is a common one, and is often the case in speech; others might include お to すすめ, "recommendation," etc. Sometimes a word will always have it as the context will always call for it. お客様 (おきゃくさま) is the polite way the customer is referred to by services in Japan, literally "customer [who is the] superior"; another is お子様 (おこさま) in menus, "child/ren [who is/are the] superior." For the sake of beginners, when seeing お and then some more stuff, especially when proceeded by kanji, one can best assume that the お is there for politeness, which is often always the case.
As for the original comment, I myself learned that sentence very early on, and the 何番 confused me at first, obviously with the redundancy ("What number is your phone number?"), but this redundancy is a common occurrence in Japanese studies. However, the sentence of the original comment is perfectly correct grammatically, and should be accepted.
The reason が didn’t work is because this is an interrogative sentence, and thus using は is more convient in that case.
To understand this, let’s start with a simple example:
アーロンは高校生です。(Āron wa kōkōsei desu.) — Aaron is a highschool student.
There is no actual partner to は in English. Due to the nature of the particle – which indicates the topic – this sentence could literally be translated as:
“When it comes to Aaron, he is a highschool student.”
In this example, the speaker knows who is Aaron. What they want to specify in the conversation is that he is a student!
Now, let’s change the particle with が: アーロンが高校生です。(Āron ga kōkōsei desu.)
The particle が directly links itself to the subject of the sentence and thus this could be literally translated as:
“Aaron is the one who is a highschool student.”
Aaron is new and important information in this case, because the speaker wants to know WHO is the highschool student!
This specification that が implies is also the reason why it’s used in sentences with 誰 (dare; Who?) because the speaker is interested in WHO will do what.
Back to your topic: we all have phone numbers, right? You are hanging out with someone, having a great time and suddenly wish to write down their phone number. So, naturally, you change the TOPIC from something you were doing to the phone number. There are many stuff you can talk to your friend about, and you choose which topic you want to talk about with は!
Hope I wasn’t too late and that this wasn’t too confusing. Just remember to learn when you make an error to keep yourself motivated!