I would change the question to "For what time is the wake-up call set?" Also, you could ask "What time does the guest request for his wake-up call?"

thanks for your answer, Phil!

Even better answer! "What time is the wake-up call?" A wake-up call is a real phone call. You ask the hotel front desk to call you at a certain time. The call is not 'set' like on an alarm clock. For your second question, slight change. Make it past tense, and again call: "What time did the guest request for his wake-up call?"

you want to asked me who guest is