I think,
you should contact Sun people directly.
I had the same issue and as far as I understood Sun has something similar to DVD regions.
There are at least three of them: 1. USA, Canada 2. EU 3. Asia and Africa Economic Region
If you purchase voucher in particular region you can take an exam in any country that belongs to the same region.
But I think that best way to make it clear is to contact the local Sun's office.
Hope this helps