You should report this activity to your state attorney general's office and the state licensing agency. Normal collection agencies are allowed to call once a week at work unless you tell them not to call you at work.
You will need to consult with a consumer protection lawyer locally.
1. Start keeping a detailed log of all calls and letters. You should also demand in writing all communications from creditors is in writing.
2. Do not give them any personal information because that is how collectors decide on which accounts to recommend suing.
3. 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.
4. All collections are negotiable.
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
5. Get written confirmation of any payment plan the agency will accept.
I do not practice in your state and you will need to consult with a local lawyer for protection under your state law.
I have pasted a link to the FDPCA to help you with your federal rights;
I hope this information and generic advice is helpful.