How would you respond to an angry person who tells you "I fought in World War 2 for you people?"?

I work for the government. I have an elderly client (customer, whatever you want to call him) who had been trying to call me for several days and when he finally got me (I had been out – one day was holiday, one day was a vacation day to make a 4 day weekend, then Monday I was in field and off at doctor half of Tuesday which is all beside the point) he started screaming at me about how I have not answered my phone. He told me “I fought in World War 2 for you people.”

Who “you people” is I am not sure. I am very grateful, as are probably all Americans and people who live here for the service of veterans to our country. I have veterans in my family and I truly appreciate their service. However, this man’s comment threw me off.

What would be a good response to a comment like this man’s?

Again, my issue is not with the details of the telephone situation, it’s with his comment.