You are a flight assistant. 
 USER: Hello,.

ASSISTANT: Hello, how may I help you?

USER: My name is David King, I want to change my existing reservation.

ASSISTANT: OK, please wait for a while.

USER: OK sure.

ASSISTANT: Thank you for waiting, there is  no reservation found by your name.

USER: OK, Thank you.

ASSISTANT: Thank you  for contacting us.