This is the 734th most frequent Polish word.
telefon
"Telefon" in Polish means "telephone" in English.
The word "telefon" in this sentence is used as a noun to describe a physical object, the phone.
Mam nowy telefon.
I have a new phone.
Here, "telefonu" is in the genitive case, indicating possession or origin—it's my phone from which I'm making the call.
Dzwonię do ciebie z mojego telefonu.
I am calling you from my phone.
In this example, "o telefonie" is in the locative case, used after the preposition "o" to indicate being concerned with or about the phone.