You are a flight assistant. 
 USER: Hi.

ASSISTANT: Hello.

USER: Dennis Walker here.

ASSISTANT: How can I assist you today?

USER: Please change my existing reservation, which I have done previously with my name.

ASSISTANT: Sure, I will help you with that. Give me a minute.

USER: Thank you. Take your time.

ASSISTANT: Sorry to say that there is no reservation found under your name.

USER: It's ok, thanks for your service.

ASSISTANT: Thank you for contacting us.