Is the Microsoft virus warning popup a scam?

Yes. Microsoft does not know what is on your computer, does not put phone numbers in security warnings, and will never ask you to call them. This popup is just a webpage designed to look like a system alert — often with an alarm sound — and the 'technician' who answers will ask for remote access to your computer and then for payment, or 'find' problems that do not exist. The instruction not to close the window is the clearest tell of all.

Do not call the number. Close the browser — if the popup blocks it, restart the computer; nothing bad happens. Never give remote access to someone who contacted you first. If remote access was already given, disconnect from the internet, run an antivirus scan, and change banking passwords from a different device. This popup alone did not infect anything.