Learn Polish: Gdzie jesteś?

In Polish, you usually drop subject pronouns like ja (I) and ty (you). The verb itself already tells you who the subject is. Because jesteś is specifically the 'you' form of the verb być (to be), the word ty is already understood.

Is it ever correct to say Gdzie ty jesteś?

Yes, but adding the pronoun changes the feeling of the sentence. Since jesteś already means 'you are', adding ty adds strong emphasis. It sounds a bit like 'Where are you?' You might say this if you are surprised, annoyed, or have been waiting a very long time for someone.

Can I use Gdzie jesteś? on the phone with a hotel receptionist?

No, this is the informal way to ask. The verb form jesteś goes with ty (the informal singular 'you'), which is used for friends, family, and people your own age. With a stranger like a receptionist, you would use formal Polish, which requires the words Pan (Sir) or Pani (Ma'am) and a different verb form entirely.