Never ever call it!
Oh no! This looks legit, it says Apple on it and my Facebook stuff might be at risk!
Let me repeat that: never ever call it!!
You want to, because hey, it was helpful enough to tell your computer has a problem. If that dialog knows your computer has a problem it totally knows who can help you fix it, right?
Wrong. Oh so wrong. So wrong it burns us. This is a tactic to get at your wallet. Commonly called “scareware” it is a way for unscrupulous folks to cause a panic and then help resolve that panic for a small payment. Even though it turns out there really isn't any reason to panic at all.
Some of these warnings are easy to see through:
Because of course I have a system 32.exe failure. On my MACINTOSH.
But some of them are a bit harder to distinguish:
A popup warning me my computer might be infected with pop-ups. Inception!
Here's the thing: There's no legitimate warning that will ever come from your web browser about anything but your web browser; it has no way of knowing. You're more likely to get an alert from your microwave that the ice maker in your freezer isn't working. If you already know this and you click the “OK” button, you might find that it pops right up again immediately, so you can't even close the window that is hassling you in the first place, confirming that you do in fact need to call (and pay) to make it go away.
Nope. Still false. Here's how you eliminate the issue (which is equally aggravating, just less common, on iOS):
On your Mac, turn off your wifi and close the dialog and the tab for the page that threw the warning. Then restore wifi and you should be back in business.
If for any reason that fails, force quit Safari, and then start it up again with the shift key held down. This will cause it to reopen without any windows or tabs from your previous session. That keeps this scareware site from reloading, bypassing the maliciously crafted page.
On your iOS device, put it in Airplane Mode and close the dialog and offending tab, then take it back out of Airplane Mode.
Hopefully you will have restored order to your computer, and by reading this article you have saved yourself at least some time and aggravation, not to mention money since you now know better than to believe everything you read on the Internet.