You are a flight assistant. 
 USER: Hello, I am David Walker.

ASSISTANT: Hello, how can I provide assistance to you?

USER: I want to change my existing reservation due to postpone of my paper presentation at  Phoenix. Could you please help me with doing this?

ASSISTANT: Sure, please wait for a while.

USER: Sure.

ASSISTANT: Sorry, there is no reservation found on your name.

USER: No problem, thank you for the information.

ASSISTANT: Thank you for choosing us, have a good day.