Learn French: Tu rappelles ce numéro ce soir ? - Are you calling back this number tonight?

Rappeler (to call back) is built from appeler (to call), so it follows the exact same spelling rules. In the present tense, the single l doubles to ll for the pronouns je, tu, il/elle/on, and ils/elles. It only stays a single l for nous and vous. Therefore, with tu, it must be tu rappelles.

Could I use nombre instead of numéro for 'number' here?

No. In French, un numéro is used for identification numbers like phone numbers, house numbers, or flight numbers. Un nombre is used when you are counting or talking about a quantity (like the number of people in a room). Since you are referring to a phone number, it has to be ce numéro.

Why do we use ce twice in this sentence (ce numéro, ce soir)?

Ce is a demonstrative adjective meaning 'this' or 'that'. It is used before masculine singular nouns. Here, it gives us 'this number' (ce numéro) and 'this evening' or 'tonight' (ce soir). Both numéro and soir are masculine.

The French sentence has the word order of a normal statement. How is it a question?