Although I would usually refrain from such language, I agree with the previous rebuttal. Tell these folks to go f*** themselves. Don't pay them a dime and don't give them any information.
DO NOT buy into any claims they may make regarding prosecuting you criminally. Legitimate debt collectors are NEVER this aggressive, and NEVER ask for a letter of apology! Shady collectors like to use the threat of jail when there is actually NO basis for prosecuting you based on this type of debt.*
Next time they call, ask them why you haven't yet received a summons and complaint, seeing as you were told they "already had a case going." Let them know that, since they claim to have a case in the works, you'll go ahead and let this play out in court, where you can countersue for illegal debt collection practices.**
See how that plays out. If they continue to harass you, tell them that you need some paperwork showing how much is owed so that you can make sure you pay the correct amount on the correct account. If they actually send you something, it can likely be used as documentary evidence in your favor should you actually have to go to court.
The next step would be a cease and desist letter. If that doesn't work, you can try filing a harassment charge with your local police. If the police won't take the complaint (they may tell you this is a purely civil matter), your last resort is to file for a protective order/injunction through your local court.
The asterisk references above are to pre-emptively defend my statements from attack by other readers who may be familiar with the laws I'm talking about:
*In some instances, "check into cash" type places can prosecute a non-payer based on bad-check laws. The situations and jurisdictions where this is both allowed and actually practiced are very limited. And from this complaint, it doesn't appear that this is alleged to be a "check into cash" transaction.
**The FDCPA, in most respects, applies only to collection agencies, not original creditors. However, most states have laws that regulate the collection activities of first party creditors, and all collectors are bound by harassment, assault, and defamation laws. Further, the collector in this case may actually be a third party.
Best of luck!