Two problems here:
The Chapter 13 BK was dismissed, so unpaid debts that may or may not have been included in it liiklely could not have been discharged. I am guessing this collection attempt is legal.
If the debt truly was a student loan, then it probably was not eligible for discharge in the first place. It may have been deferred during the chapter 13 proceeding. Hard to know at this point.
Given how old the debt and bankruptcy are, I am not surprised that they are not on your credit reports. Both should have fallen off some time ago. Derogs generally stay on credit reports for seven years. A bankruptcy filing stays 10 years. By your own account this debt and the bankruptcy filing are over 10 years old.
However, I suggest that you take your paperwork to a local BK attorney and get an opinion on whether this debt truly qualifies&n... a student loan. Let's just say, I would never trust what a debt collector sayson such a subject without verifying it seven ways from Tuesday. If it is not a student loan, it potentially would be eligible for discharge in chapter 7 bankruptcy since it has been long enough since your last one. If it is, you may be in a pickle, but you might be able to pay it back via another chapter 13 bankrutpcy, which likely will be hard, but less unpleasant than bank account levies and wage garnishments.