Okay, if you are talking about www.palmerpay.com, it looks like ordinary collections.
Get a recording application on your phone and record the calls. If they threaten jail for failing to pay, take the recording of the call to an attorney who has experience with debt collection actions. We're easy to find.
I'm not your attorney; my answer includes assumptions. If you want me to be your attorney, I'm easy to find.
This sounds like a collection agency making illegal threats. The Federal Fair Debt Collection Practices Act prevents collectors from making threats of arrest. More information is needed but you can generally never be arrested for failing to pay a debt. If they sued you and you were held in contempt of court for disregarding a judicial order, then it would be possible but still unlikely.
If you are paying anything to a collection agency you should have it all in writing. Some of these collection agencies will take your money and not keep a record of any payments.
My comments are not legal advice and are for informational purposes only.