All of these kind of pop-ups/alerts are fake warnings – scams. Never, ever call these numbers!
Tech Support Scamming through unsolicited phone calls, browser pop-ups and emails from so-called Support Techs advising “your computer is infected with malware”, “all your files are encrypted” and other fake messages has become an increasing common scam tactic over the past several years. The scams may involve web pages with screenshots of fake Microsoft (Windows) Support messages, fake reports of suspicious activity, fake warnings of malware found on your computer, fake ransomware and fake BSODs all of which include a tech support phone number to call to fix the problem.
ANY unexpected web page or popup that asks you to call a phone number or enter your email login and password should be considered suspect.