Learn Dutch: Dit nummer is niet mijn telefoonnummer. - This number is not my phone number.

You use geen to negate unspecific things, like a noun with een ("a") or no article at all. However, because you are talking about a very specific thing—mijn telefoonnummer ("my phone number")—you must use niet.

Why is it dit nummer and not deze nummer?

In Dutch, nummer is a het word (het nummer). When saying "this" for a singular het word, you must use dit. If it were a de word, you would use deze.

Why does niet go right before mijn telefoonnummer?

When you use niet to negate a specific noun phrase (like one starting with a possessive word like mijn), it goes directly in front of that phrase to show exactly what is being negated.

Why is telefoonnummer written as a single word?

In Dutch, compound nouns (words made by combining two or more existing nouns) are glued together without spaces. So, telefoon (phone) and nummer (number) become the single word telefoonnummer.