They cannot threaten you with jail time. That is a violation of the Fair Debt Practices Act. You need to contact a consumer attorney immediately. You have rights that need to be protected and not only could you maybe get the debt discharged, but get money from the debt collector as well for the violations.
This is not legal advice nor does it create an attorney-client relationship. This is for education and informational purposes only. It is always recommended that you contact an attorney with any concerns as each individual case is unique.
You cannot be jailed for a civil debt. Making such a threat is a violation of the FDCPA, among other things.
Call a local consumer law attorney; there are statutory penalties and attorney fees available.