I had this happen today! A similar pop up blocked my Safari and tells you Safari Suspicious Activity alert with a toll free number to call. I wasn't too alarmed at first, however, once I was told to hook my phone up to the computer that they wanted to do a screen share to diagnose issue, I started asking questions. They gave me a URL that is actually a meeting or screen share service and did not include 'apple' in it. I asked why they couldn't just walk me through the fix, since this is likely a known issue; I should be able to be walked through the fix via phone and look in settings and resolve. They quickly hung up. I came to apple support and found this page of similar scenarios and found the resolution.
Go into settings>safari>clear history and website data.
Hope this helps anyone else that experiences this issue. I was pretty surprised having the iPhone that this happened!!
It is definitely a phishing scam. They did not ask me for a credit card. They never got access to my computer, I asked too many questions. Please be careful, I had no idea this wasn't apple support from the pop up, I just wanted my phone to work properly and figured out pretty quick this was a scam.