No rule for this one
No ending or word group predicts the article of Telefon. Learn it as a pair and say it out loud: das Telefon.
das Telefon
It's das Telefon. Telefon is neuter, plural die Telefone.
Das Telefon klingelt.
The phone is ringing.
No ending or word group predicts the article of Telefon. Learn it as a pair and say it out loud: das Telefon.