Unfortunately, what you are describing has become very commonplace in the mortgage industry. There is a great deal of confusion and communication breakdown, due in part to the internal problems inside the mortgage lenders and services, and further complicated by the huge numbers of delinquent loans.
You can tell them when they call that they are not to call you again, and that you are asserting your rights under the Fair Debt Collection Practices Act. If you do that, make a note of the date, the time and tne hame of the person on the other end, and follow it up with a letter via certified mail, return receipt requested, that says the same thing, and make sure to save the receipt and a copy of the letter.
However, be warned: if they are still calling, it is likely that their computer shows that your loan is delinquent. These places are operated by their computers, and if the loan is shown as delinquent, odds are that they will start foreclosure. If you have documentation of the fact that the shortage was made up, you should refer to those facts as well in your letter, and include a copy of the documentation. They may still continue to call, but if they do, at that point they are violating the law and there is something you can do about it. They may start foreclosure anyway. If they do, do not try to handle it by yourself. You need an attorney who is knowledgable in foreclosure defense. Foreclosure defense is a highly specialized area of legal practice. There are very few attorneys who are really comforable with how it works, and who understand who the various players are and what can be done to handle these situations for borrowers.
As things get crazier in the mortgage industry, it becomes more and more difficult for borrowers to get anywhere with the lenders and servicers, and often the only way is in court. While it should not be this way, unfortunately the mortgage servicing process has fallen apart. They don't have enough people there to even take the calls, let alone to address issues with borrowers. I do foreclosure defense in Florida, and have success getting past the stonewalling because I do it in court, and because in court if the lender has not done properly what it is required to do, the court can stop them from having their way. However, every state is different, and what would need to be done by an attorney in your state is very different than here.
The National Association of Consumer Advocates (NACA) is a non-profit consumer advocacy organization. NACA maintains a web site at www.naca.net where it lists geographically consumer law attorneys all over the US. If you don't already have a foreclosure defense attorney, please look there for someone in your area who specializes in foreclosure defense to review the details with you and advise you.
One thing you may want to check is whether you paid all the fees associated with the late payment.
You appear to have at least one late payment. If so, you likely were charged all sorts of fees. If you do not pay enough to pay off those fees, your monthly payment would first be applied to the fees, resulting in another late payment and more fees.
You likely will need to look at the statements from the company to find out why the company thinks you are behind.