Questions & Answers about Откуда у тебя телефон?
The structure у + genitive pronoun expresses possession in Russian.
• у тебя = “you have”
It literally reads “at you,” indicating that the phone belongs to you. You must use the genitive case after у, so тебе (dative) would be incorrect here.
It’s omitted for brevity and is common in Russian. The full version would be:
Откуда у тебя (есть) телефон?
In spoken and informal written Russian, есть can drop out when talking about what someone has.