Well, normely you get a pop-up link with something like "HEY ASSHOLE!!! YOU HAVE A VIRUS. HERE IS YOUR CUSTOMER ID: 4563.
You can't just call a number, you gotta have the scam link open for them to believe in you. I hope this was helpful ;)
Btw, if you wanna avoid the id, just say that you cleared the history when you restarted the computer (it "froze") so you "had no choice".