Yes, you could call FB, though the FB website wants you to send them a fax. Yes, it's 2010 and they want you to send a FAX!

If you call and you're in the UK, don't use the ripoff 0870/0871 number. It will probably be cheaper if you call +33 1 5868 6868 (Paris) and select the English language option. My call was picked up in Holland by an English-speaking Dutch person.

The advice they gave me when I asked the same question was to make the booking first, explaining that you will be using an FB certificate. Once you have the reservation, fax/call FB. In my case, the request was actioned immediately and the voucher came via snail mail.