[Resolved]  HDFC Bank Home Loans — Principal Balance of home loan

My parents acquired a home loan from Centurion Bank of Punjab Limited - for :

Term: 180months
Amount Sanctioned: Rs.12, 59, 868/-
Rate of Interest : 10.00% floating
Date: 6th April 2005

This loan was then sold to HDFC bank home loans and after paying this for almost 4 years with monthly payment for Rs.13224/- - we receive letters showing our principal amount as:

Term: 257 months
New EMI: Rs.13833/-
Principal Remaining Balance: Rs.11, 68, 954-
Interest Rate: 12.75% floating

we have been getting collections calls, which are like harassment because we are never late on pymnts and also they dont clear anything as to why tem is increasing but amount of EMI is high and principal balance is increasing!
What should be our immediate step?

We have also ordered for transaction history, but what should we do about the principal balnce? They havent cleared anything on that?