Learn Arabic (Modern Standard): ما رقم الهاتف؟

This is the core rule of the Idafa (possessive) construction! An Idafa links two nouns together to mean "the [noun 1] of the [noun 2]". The first word (رقم) never takes the word for "the" (الـ). Adding the definite article to just the second word (الهاتف) is enough to make the entire phrase specific.

Why is the word order رقم الهاتف (number the phone) instead of matching the English "phone number"?

In Arabic, when combining two nouns to show that one belongs to the other, the "possessed" item always comes first, followed by the "possessor". Since it is the number belonging to the phone, رقم (number) goes first, followed by الهاتف (the phone). It helps to read it literally as "the number of the phone".

Why do we use ما (mā) for "what" here instead of ماذا (mādhā)?

You use ما (mā) to ask "what" in sentences that do not have a verb, such as when asking about a noun (a name, a number, a color). You use ماذا (mādhā) when you are asking "what" alongside an action verb, like "What are you writing?".

Where is the word for "is" in this sentence?

Arabic does not use a word for "is" or "are" in the present tense. You simply put the two parts of the sentence next to each other. Placing ما (what) right next to رقم الهاتف (the phone number) automatically means "What is the phone number?".