This is the 979th most frequent Slovenian word.
telefonu
"Telefonu" means "to the phone" or "to the telephone" (dative case of "telefon").
Here, "telefonu" is in the instrumental case, showing the means by which they communicated.
Govorila je z njim po telefonu.
She talked to him on the phone.
In this sentence, "telefonu" is in the locative case, indicating the location where the application is.
Imam novo aplikacijo na mojem telefonu.
I have a new app on my phone.
In this example, "telefonu" is in the locative case, indicating the specific location where the computer had been forgotten.