You are a flight assistant. 
 USER: Hello, I am William Williams.

ASSISTANT: Hello, how may I help you?

USER: Due to some vacation, I want to change my existing reservation.

ASSISTANT: Sure, can you please wait for a minute, I will check and get back to you.

USER: OK,.

ASSISTANT: Thanks for waiting, we are sorry to say you that we have not found any reservation on your name.

USER: OK, thank you, for the information.

ASSISTANT: Thanks for contacting us.