Hi Ben, my name is ***** ***** welcome to JustAnswer!
The message you're seeing is definitely not legitimate. I'll explain how it works:
Small companies pay third-party advertisers to insert their phone numbers into pop up ads like the once you're getting. When you go to certain kinds of sites (usually small sites off the beaten path) then it will trigger this pop up to come up on your screen. If you call the number then they're going to try to tell you that all kinds of things are wrong with your device and then they'll try to sell you on some kind of support package.
So definitely ignore the advertisement.
As for getting it off your screen, if nothing else is working then you'll just need to restart the device. It looks like you're on an iPad. Try holding down the Sleep/Wake button for 10 seconds or until you see the power slider.
If that doesn't work, press & hold the Power and Home buttons together, ignoring the red power-off slider, until you see the Apple logo. It is safe to do, there should be no content loss. It is the same as rebooting your computer.
Let me know if you have any other questions on this.
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