Compound nouns: the last word decides
Telefonnummer die
Always In 98.9% of 6,721 common compound nouns, the article comes from the last word.
The last part decides the article: die Nummer, so die Telefonnummer.
die Telefonnummer
It's die Telefonnummer. Telefonnummer is feminine, plural die Telefonnummern.
Wie ist Ihre Telefonnummer?
What's your phone number?
Telefonnummer die
Always In 98.9% of 6,721 common compound nouns, the article comes from the last word.
The last part decides the article: die Nummer, so die Telefonnummer.