No rule for this one
No ending or word group predicts the article of Anruf. Learn it as a pair and say it out loud: der Anruf.
der Anruf
It's der Anruf. Anruf is masculine, plural die Anrufe.
Ich erwarte einen Anruf.
I am expecting a call.
No ending or word group predicts the article of Anruf. Learn it as a pair and say it out loud: der Anruf.