You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello. How may I assist you today?

USER: I want to change my existing reservation. Could you help me with that?

ASSISTANT: Sure I will help you. Please help me with your name.

USER: John Walker here.

ASSISTANT: Please allow me a minute.

USER: Ok.

ASSISTANT: John Walker, I am really sorry to inform you that there is no booking found on your name.

USER: That's ok, no problem. Thank you.

ASSISTANT: Thank you for reaching us. Have a great time!.