Where and when are you trying to fly using the free round trip option? If it is during peak travel times and/or a popular destination, then the chances are that the seats available for purchase, UNDER THE VOUCHER or the frequent flyer program, are gone. But seats are still open for purchase and you should be able to use the voucher for $200 off that fare.
And I agree with PHX, there is no reason to call the executive offices to redeem a voucher. They don't do reservations. Maybe you were directed to call them because you were unhappy with the inability to use the free round trip option.
If you still want to use the voucher for $200 off your trip and reservations can't help you, send us a message at
http://airconsumer.ost.dot.gov/problems.htm and one of our analysts can try and help.