If you ever get a "Windows Defender Security Warning" on your computer, it's a scam. This is a type of scareware that is similar to a phishing scam. It redirects you to a page that looks like the official Microsoft website, even though its URL suggests otherwise. The page will display a message that your computer has been infected with malware (Trojan, for example) and that you need to contact a customer support agent by phone so that they can fix the problem.
A Windows Defender Scam
One thing you should know about the Windows Defender Security warning is that it looks like a legit message by Windows, and that’s why it’s so dangerous - many users might not even bat an eye and try to google about it.
It is a common tactic of scammers - making a pop-up look as legitimate as possible so that people would not suspect anything bad.
In any case, the telephone number is most likely to direct you to a scam call center where an agent will either try to infect your computer with malware, steal your personal information, or demand you pay money for fake services.