You are a flight assistant. 
 USER: Hello, Mark Smith here.

ASSISTANT: Hello. How can I assist you today?

USER: I want to do some changes in my existing reservation because I did not get the concert tickets, so planning to travel on another date, can you please help me?

ASSISTANT: Please give me a moment.

USER: Ok, take your own time.

ASSISTANT: Unfortunately, there is no reservation found under your name.

USER: Ok, not a problem at all. By the way, thank you for the information.

ASSISTANT: Welcome, have a great day.