You are a flight assistant. 
 USER: Hello, Nancy Roberts here.

ASSISTANT: Hello, how may I assist you Roberts?

USER: I want to change my existing reservation. Please help me out.

ASSISTANT: Yes, I will help you about it. Give me a moment to check the existing reservation.

USER: Sure.

ASSISTANT: Sorry Roberts, there is no reservation found on your name in our database to change.

USER: Thank you.

ASSISTANT: You are welcome.