I've been called Fortnoy many times from people in my generation or older. I put the blame on Philip Roth's book Portnoy's Complaint.
My father's birth name was Paul Fortunow. The name has Russian roots, possibly connected to Fortunoff. The 'u' in Fortunow is silent. Go ahead and try and pronounce Fortunow. You forgot the "u" was silent. So a couple of years before I was born (and before Portnoy's Complaint) my father changed his last name to Fortnow.
Foreigners usually have no trouble pronouncing my name. But once when I was checking in at the Frankfurt airport, the agents said something like "Here are your tickets Mr. Fornow". I said "Thanks but that's Fortnow". She responded "If it was pronounced Fortnow there would be a 'u' after the 't'". I should have asked her where she was from.