This is the 477th most frequent Czech word.
telefonu
"Telefonu" is the genitive or dative singular form of "telefon," meaning "of the phone" or "to/for the phone" in English.
In this sentence, 'telefonu' is used as the genitive singular form of 'telefon'. It indicates possession or association, meaning 'of the phone'.
Požádal mě o opravu telefonu.
He asked me to fix the phone.
In this case, 'telefon' is used in its nominative (direct object) form, indicating the phone itself as the object of forgetting.
Zapomněla jsem telefon doma.
I forgot my phone at home.
Here, 'telefonu' is again in the genitive singular form, used to indicate 'of the phone' in a descriptive context.