This is the 384th most frequent Lithuanian word.
telefonu
"Telefonu" means "by phone" in English.
In this sentence, 'telefonu' functions as the means by which the action of talking is performed, therefore it is in the instrumental case.
Jis kalbėjo telefonu visą dieną.
He talked on the phone all day.
Here, 'telefonu' denotes possession and refers to 'my phone,' used in its standard form in Lithuanian.
Mano telefonu nesugedo.
My phone did not break.
In this context, 'telefonu' again signifies the means, indicating that the meeting will take place via phone.