While I don't work in the field, first line of attack IMHO is to contact one's health insurance company--assuming one had coverage at the time of the incurred charge. Even if it was from several years ago, if it's within the CRTP (or SOL in most states) then odds are good the insurance company will have docs. They are incredible record keepers.
FCRA and FDCPA violations are subject to the opinion of a judge, but a provider violating the rules of an insurance company doesn't have such protections or vagueness in the law. When the insurance company speaks, providers listen and act.
Anecdotally speaking, from talking to folks, a great many medical collections come about because the provider failed to fill in a box on a paper form correctly, filled it in with something the insurance company could not decipher, or the provider failed to respond to the insurance company's request for more info.
If the provider was in network, they will sometimes try to bill for charges the network plan will not PERMIT the provider to bill. I know this from personal experience. Happened to me several times in recent years. I've had to get on the phone to the insurance company, ask about the claim, the charges, what was paid, what was denied, and of what portion am I responsible. While sometimes appearing to be a little cold and clinical, I have always had good experience when I call. They give me the details I ask about.
Sometimes I've taken what I learned, called the provider, and problem fixed. A few times, the insurance company rep has offered to call and talk to the provider directly. I think I had 'em do that once or twice, but I try to handle it myself with the provider so as to try and keep on friendly terms with the provider.
Have ready the date of service, name of the provider, SSN and name of the insured, SSN and name of the patient (although SSN might not be needed). Also DOBs for both the patient and the insured.