You're calling at a bad time.

I'm sorry, I have to go. Could you call me later?

I'm sorry, I'm busy now. I'll call back later.

It's not a good time. Could you call again in about 10 minutes?

전화 받기 어려울 때 / 다시 전화주시겠습니까?

Just feel free to call.

Please give me a call anytime.

If you have any questions, you can call me anytime

언제든 전화주세요.

How soon do you expect him back?

When do you think he(she)'ll be back?

Can I ask when he'll return?

언제 담당자와 통화가 가능할까요? 

He is (speaking) on another line.

그는 지금 딴 전화를 받고 있습니다.

I'll see if he is in.

그 분이 안에 있나 알아보죠.

You are wanted on the phone.

There's a call for you.

네게 전화 왔어.

Answer/Get the phone.  전화받아

 Where are you calling from?  전화하는 곳이 어디죠?

Is this 00 company?  거기 00 회사죠?

Who shall I say is calling?  누구라고 전할까요?