Sorry, loan company. They need to be able to PROVE the ORIGINAL debt, not the phone thing. If they cannot prove the ORIGINAL debt, they cannot collect. They can attempt to go to court and sadly they might win. WHY? Most people don't show up to contest. most people who do show up do not have records of each conversation (be SURE to document name and ID info of person you spoke with, some co's won't give full names but if they don't they have to give an ID number of some kind, not just "Pam" or "Mr. Smith") or of the debts and how much was paid on the original debt.
IF you go to court and show you have asked them to PROVE the original debt with the original loan, and they cannot? Usually you walk out with-o owing a penny - they even pay court costs.
Your next move? Tell them they need to show you proof of the ORIGINAL debt, not some agreement you entered under duress. The duress was their threat of jail time and it makes the agreement invalid. They specifically called you at work, threatened to send someone to serve you or arrest you there, and they threatened you with prison time to get an agreement. This is illegal. SOme judges would throw out the entire amt owed just for that. I know, because I had a judge do this. I was the sixth case with the same creditor where one payment two days late ended up in court. NO clue how they even got it filed, but they tried to make sure I didn't know when the court date was, and I PROVED that they put the wrong addy on the notice and since it was done in every other case, plus they threatened all sorts of stuff, the judge was angry with them.
If they cannot show that you owe the original debt, how can they collect? Tell them your bank is investigating and it will take time, that you need to see the original paperwork on the debt and unless they can provide it, you will be waiting to speak to your bank and will consult an attorney. Again, tell them that they are NOT to call you and that ALL communication must be in writing so that there are no more misunderstandings or threats of prison etc.... Tell them that if they call you again, you will file a complaint with the Feds.