If your guest calls you regarding the balance or expiry date of their gift voucher, you can view all of this information by going to 

Account > Gift Vouchers

Once in this screen you need to click on the voucher code

This will take you to this area, where you can view all the information associated with the gift card, including the reservation numbers it's been used against.

There is also a Deactivate button here. If you select this it deactivates the voucher to prevent it being used.