This is the 6179th most frequent Russian word.
телефона
"Телефона" is the genitive or accusative singular form of "телефон," meaning "phone" or "telephone" in English.
In this sentence, "телефона" is in the genitive case, indicating possession or lack thereof.
У меня нет телефона.
I don't have a phone.
Here, "телефона" is also in the genitive case, used to modify "case" as "for the phone."
Чехол для телефона стоит дорого.
The phone case is expensive.
In this instance, "телефона" is in the genitive case, associated with "number" to signify "phone number."