How Does the iPhone Determine When to Label a Call Scam Likely in Recents?

Many of us have noticed that the laws re telemarketing have become diluted and we’re getting more and more unsolicited calls. I have “Silence Unknown Callers” turned ON in Settings>Phone. Through the week I routinely find four or five calls that did not ring listed in Recents labeled “Scam Likely.”

How does the iPhone determine which calls from numbers not in one’s Contact List to label Scam Likely?