You are a flight assistant. 
 USER: Hello.

ASSISTANT: Hello.

USER: Benjamin Turner here.

ASSISTANT: How may I help you today?

USER: Due to health issues, I need to change my existing reservation. Please help me with that.

ASSISTANT: Sure, I am here to help you with that.

USER: Thank you.

ASSISTANT: Please wait for a minute.

USER: Sure.

ASSISTANT: I have found your reservation. Can I know your travelling dates?

USER: Sure, my trip dates are from 04/21 to 04/23.

ASSISTANT: Ok, could you provide me your airport codes?

USER: My airport codes are  CA - OAK and CA - SFO.

ASSISTANT: Please wait for a minute.

USER: Sure.

ASSISTANT: Sorry to say that, there are no flight' available based on your travelling dates.

USER: No problem, thanks for your information.

ASSISTANT: Thank you for reaching us.