Question to an unknown caller crossword clue

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I must admit that this clue is not so easy. It took us some time to find the right answer to Question to an unknown caller. You already know the answers has 9 letters. If you still cannot find it, here it comes.

Clue: Question to an unknown caller
Answer: WHOISTHIS
Now you know the answer to Question to an unknown caller. It’s time to go back to find the answers to others clues on Premier Sunday December 23, 2018

Wrong answer?
Sometimes the same clue can have 2 different answers or more. For example, the clue “Like some funds” could have 3 answers: MUTUAL, MONETARY, ALLOCABLE found respectively on the following crosswords: USA Today (April 22, 2015 and April 30, 2010) and New York Times (Oct. 27, 2000)