You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, may I know your good name please?

USER: My name is Anna Lewis.

ASSISTANT: What can I do for you today?

USER: I want to change my existing reservation due to postpone of Christmas shopping.

ASSISTANT: Ok, please wait a minute. I will check your reservation.

USER: Sure.

ASSISTANT: Sorry, no reservation has been registered with us.

USER: No problem, thank you for your service.

ASSISTANT: Thanks for approaching us, have a nice day.