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"?
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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?"