Nashua and creditworx

I had a nashua contract which i paid every month, very well. Nashua closed down in November 2014. I made payments to nashua mobile till december. But now i receive endless calls from creditworx saying i am owing nashua mobile for december. I have faxed proof of payments to credirworx hope this will help out. If this continue, I am taking this matter to court because I cannot be ********** just like that. I am sick and tired of this!!!