I do not practice in your state. The interest will be on the original agreement for medical service that was signed when you went in. Most states allow interest on judgments if it has gone that far already.
You will need to consult with a consumer protection lawyer locally.
1. Start keeping a detailed log of all calls and letters and a paper file of all information.
2. Make a written demand that all further communications from creditors is in writing under 15 USC 1692 (c).
The letter should also contain a dispute of the validity of the charges and include a demand for a complete accounting with signatures, and all contents of the file.
The creditor then has 30 days to reply and they may not take any action until you have been sent the validation. Bear in mind that this may be motivation for the collector to work your account when the file comes to them from the original creditor with new information.
3. Do not give them any personal information because that is how collectors decide on which accounts to recommend suing.
4. If you are going to make payments use money orders and not personal checks or “check by phone” because if they find a bank account the collector will be more likely recommend a lawsuit the their legal department.
5. All collections are negotiable; the original creditor has given up and is losing up to 50% on the face value already either by splitting any return or selling at a huge discount. In addition, the costs of a lawsuit although discounted still are a factor in the decision to settle with you.
If you are going to settle mark the check “settled-in-full” at the very top back of the check and include a letter explaining you are offering a settlement, keep copies of everything.
6. Get written confirmation of any payment plan the agency will accept before making a payment.
7. Specify in writing all payments will be applied to principle first.
I do not practice in your state and you will need to consult with a local lawyer for additional protection under your state law.
I have pasted a link to the FDPCA to help you with your federal rights;