He is not in right now.

He is out at the moment.

He just stepped out. (방금 나갔습니다.)

I'm sorry, but he(she)'s out.

찾는사람이 부재중일때....

May I take a message?

May I have your message (for him)?

Is there any message?

I'll be glad to take a message.

Would you leave (me) a message?

Would you like to leave a message?

Do you want to leave a message?

Could I tell him(her) you called?

전하실 말씀 있으세요?  

What's this in regard to?

Regarding/Concerning what?

What's this about?

What's this regarding/concerning?

용건이 뭐에요?