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Definitely a scam. Do not call that number.

Force quit Safari using the Command + Option + Esc keyboard shortcut. Then relaunch Safari while holding down the Shift key.

Then relaunch Safari. From your Safari menu bar click Safari > Preferences then select the Privacy tab then click: Remove All Website Data.

Quit then relaunch Safari as you normally do.

Ignore everything it says. It is all false, including the admonition not to shut down your Mac, which wouldn't help anyway.

Read Phony "tech support" / "ransomware" popups and web pages for the solution to this common scam.