It is an unfortunate and little-known fact that the Fair Debt Collection Practices Act (FDCPA) does not apply to every debt collection situation. Two requirements must be met before the FDCPA comes into play.
First, the debt that is involved must be a "consumer debt". The FDCPA defines "consumer debt" as any debt where the money was used to buy goods or services that were "primarily for personal, family, or household purposes." What does this mean in English? It means that business debts are not covered by the FDCPA. Only debts incurred to buy goods or services for use by you, your family, or in your house.
Second, there must be a "debt collector" involved. Under the FDCPA, "debt collector" is a term of art that means a business that collects the debts of another. This means that the original lender or creditor is not covered by the FDCPA. So if you had a Capital One credit card and the Capital One collection department is calling you, they are not required to follow the FDCPA. But if ABC collection agency is collecting on behalf of Capital One, the FDCPA applies to them. Law firms are also covered by the FDCPA if they regularly collect consumer debts. So the FDCPA definitely applies to a collection law firm, but probably doesn't apply to a law firm that only occasionally handles consumer collection cases.
Of course, even if your situation involves a consumer debt and a debt collector, there must still be a violation of the FDCPA.