Recently I’ve been reading a tech site on my iOS device when a new tab opened and went into the background - something I’ve seen before caused by rogue ads that are fetched via normal means (e.g. embedded in a legit page), so my next steps were not unusual - go to the tab and close it. Clearly they want to automatically call whatever number it is programmed to call, which I can only assume will go to one of those call centers where you will be asked to provide some private information to unblock the phone from some scary virus that might be stealing your info from your iPhone.