I bought a UA flight using a paper voucher at the aiport. Airfare: $399-$300 voucher cost me $99. I now see the flight is selling for $80 less. I can't refare online, because it says my itinerary cost was $99 and there is no cheaper fare available.

Before I call and either get a unknowing agent or make a fool of myself, can I take care of this on the phone? Is there a record in my PNR of me using a certain value voucher?

thanks.