Un-credited Payment

After my account number was changed by ACS, I continued to make payments as usual through online banking. After getting dunning letters, I discovered that the account number had been changed. All but one of the payments has been credited to my account. I have mailed in proof of payment twice to no avail. I have tried calling the apparently un-manned 800 number and have been on hold for several hours several times. It would appear that there is no way to speak to an individual to find out why the payment has not yet been credited. my account number isRita Newcomb