You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello, may I know how best I can help you today?

USER: I am Charles Phillips. I want to change my existing reservation. Could you help me with that?

ASSISTANT: Please wait for a while.

USER: Ok.

ASSISTANT: Your reservation found, can you please share your details.

USER: My travelling dates are from Dec 12-14.

ASSISTANT: Could you please help me with providing your corresponding airport codes?

USER: My airport codes are IAH to LAS.

ASSISTANT:  No flights are available on respective dates of travel.

USER: That's ok, no problem. Thank you.

ASSISTANT: Thank you.