Translation:What is your phone number?
電話(でんわ) 番号(ばんごう) を 教えて(おしえて) ください is more commonly used and sounds more natural tbh since it's an actual request and polite. It translates to: "Please tell me (your) phone number"
電話番号は何ですか is easier to teach and understand though... hence why it's taught (since cultural nuances and けいご or "formal speech"/"honorific expression" can be overwhelming). Simpler structure and relatively direct translation.
Generally nan, or nani is used for which ever flows better. However, Nani is used when wanting to be more polite and formal.
Use nan before b, p, d, t, n, r, and z. Use nani before all other letters.
There are some exceptions, though.
If nan was used before ka/か, it would sound like the word nanka, which means "things like" or "something like that" which is also considered rude.