Note: You have an IO for the first 10 years, so the only way to lower your balance is to make an EXTRA payment to principal that is above and beyond your minimum interest payment. The best source of a direct answer to your particular arrangement is with the lender. If the CS rep that you first talk to can’t help, then ask to speak to a supervisor.
Basically, it depends on when/how your mortgage calculates your interest on the principal. In the best case, your loan is re-amortized everytime you make a payment, meaning that the principal from which your interest is calculated is lower each time because of the previous payment that you made. But, this is generally only true of principal and interest payments.
In most cases, the loan is re-amortized only once a year, so you can lower your balance by simply making ONE extra payment at your anniversary date. The best thing to do is to call your lender and ask this directly. You could even request a payment option that lets you pay the payment in 2 monthly installments or on a bi-weekly basis.
