This happened once to me on Windows 10 and my husband on a MacBook Pro. (believe it or not). It looked very suspicious, so I did not call the provided number, but called Microsoft and AppleCare respectively when it happened. They were both very helpful. In the case of the PC, Malwarebytes took care of it. For Apple, I don't remember what we did, but it has not recurred on either computer.
What I took away from these experiences is not so much how to fix it, but not to panic and don't fall for it. If the number on the screen is not a number you already have in your contacts, don't call it!!

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