What are the arguments that the debt is a consumer debt?

Kevin Miller bought a house in Atlanta in 2012 and took out
a mortgage. He lived in the house until 2015, when he
accepted a job in Chicago; from then on, he rented the house.
He received a letter demanding payment from a law firm on
behalf of the mortgage company in 2017. By this time, Miller
was renting the property to strangers and thus was making a
business use of the property. Miller claimed that the law firm
had violated the Fair Debt Collection Practices Act. The law
firm replied that the letter is outside the scope of the Act
because it was trying to collect a business debt rather than a
consumer debt.
a. What are the arguments that the debt is a consumer debt?
b. What are the arguments that the debt is a business debt?
c. Which arguments would prevail? Explain.