I must go suddenly to the business trip today so I cannot answer the phone. 

Correction: I suddenly need to go on a business trip so I will not be able to answer your call.

Wish to understand although I cannot get please. 

Correction: I can't attend our class today. I hope you'll understand. 

Thank you. [good sentence] 

Message:

Hi Shawn,

Thank you so much for leaving a message here and for informing me that you will go on a business trip. I truly appreciate it.

I understand. :) I am always looking forward to more chances to talk with you.

Have a wonderful day! See you next class! ^_^

Take care, 
Teacher Jae ♥