Father being harrassed by Municipal Services Bureau

Hello,

My father is being harassed by a collection agency named Municipal Services Bureau demanding payment for a "City of Los Angeles (Business Tax)" matter. My father is a senior citizen and doesn't know what the debt is about. The date of the debt printed on the letter is 6/13/2017. What can I do to help him? Write a debt validation letter? Call the collection agency and get more info? Hire a professional to deal with this?

Thanks.