I keep getting a phone call on my iPhone with a message on top of the screen that says my phone has 15 viruses.

No, it’s a scam to steal your money or your identity. Ignore it.

See→Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

delma176 wrote:

Is this a hack?

Nope.

It’s an advertisement intended to hack its readers into doing things against the readers’ own best interests.

It’s hacking people. Us. How we think, and how we then react. Not hacking our iPhones.

Hacking us—or trying to—is easier than hacking our iPhones, too.

Scam Pop-ups, Scam Viruses, Scam Receipts… - Apple Community