Aside from th fact that I never call unknown numbers I would probably say "Hi, this is ..........., I just missed your call" or Hi, this is ..........., did you try to call me just now?"

I would say "Hello, this is xxx. May I asked who was it that just called me?"

Correction: I MISSED a call from AN unknown number AND I want to call back. How can I say it in English properly? "Hello, this is xxx. Who called me just now"? ============================================= Answer: Try saying something like this: "Hello, this is xxx. It seems like I just missed your call awhile ago. Who's this, please?" "Hi. This is xxx. I seem to have missed your call a minute ago. May I know who this is, please?" "Hello. This is xxx. I got a call from this number a moment ago, but unfortunately, it's not registered in my phone. May I know who this is?"