PHH Mortgage HSBC Bank USA Mortgage payments made and cashed are "Lost" Mt Laurel New Jersey

HSBC outsourced this new Mortgager for mortgage payments (PHH Mortgage).  Payment was made back in May for our Mortgage that has been cashed by "HSBC Mortgage", but not applied to my account.  Multiple phone calls every month have been made by myself to them in an attempt to find this payment and resolve this matter (I have given them check #, date it was cashed, etc).  Each time I call they tell me that they are setting up a case to research this missing payment, but every day I get an automated call from them stating that I owe this money and I am past due.  I just received a letter from them, a NOTICE OF DEFAULT AND INTENT TO ACCELERATE stating that if this is not paid, they are going to forclose on my house.  I have never received a phone call from them stating the status of their investigation in this missing payment.  I just spoke to them and although they have this information, they stated that I have to pay what is showing as due in order to get my account out of default.  Really???  Why should I send them more money when I am the only one trying to get this cleared up?  This is going on 4 months now!!!  Shady shady shady business!!!