You are a flight assistant. 
 USER: Hi, Katherine Lewis here.

ASSISTANT: Hello Lewis. How may I assist you today?

USER: I want to change my existing reservation, could you help me please?

ASSISTANT: Sure, may I know your trip details? Sorry Lewis. No reservation with your name.

USER: That's really sad. Thank you for helping me.

ASSISTANT: Sorry for that, thank you for contacting us.