castle credit corp. contract says 48 pymts to start 3/09 they filed breach of contract when we have 13 pymts left chicago, Illinois

Have been getting harassing calls and mail. Signed our contract for the Micron Filtration Tech Inc finaced through Castle Credit Corp on 12/06/08. Our first payment was on 03/06/09. They gave us 48 payments (which is 4 years). They want the remaining balance of $1,817.00 immediately or they are going to file legal action we have not spoke to them since February 2011. They say on the letter they sent to us that we agreed to pay the amount left owed by 04/29/11 and said they spoke to us and we agreed to the payment. But as I stated we have not spoke to them since February. We only asked for them to tell us what the payoff amount was not that we would pay it off. They are calling us about 3 to 6 times a day we are tired of being harrassed by them.