Next time they call ask what Court the lawsuit was filed in? Get name of the person calling, his company, how he got your debt, did he buy it?
Then let him know that it is a violation of the FDCPA (Fair Debt Collection Practice Act) to tell you that a lawsuit has been filed when in fact that it has not been. You may even wish to contact a local consumer protection attorney who handles this sort of case (it may be tough to find someone).
I highly doubt that a lawsuit has been filed because they are required to serve you notice of the lawsuit and it does not seem like you have received notice of the suite. Although it is possible that notice was served on you at the wrong address.
In short, do not let these guys bully you into paying and being scared. Call a lawyer if it keeps up.
A complaint filed in federal district court must be served on the defendant within 120 days of its filing in court. If you are curious, in addition to the government's PACER service, there are several commercial databases (such as Westlaw and Lexis) that track court filings. There is also a free website http://dockets.justia.com/ that you can search.