Ash,
We got something similar from Aer Lingus last year, it said that we MUST ring the aer lingus booking line to use the voucher. So after paying an extra 10 euro or so for the hassle of trying to explain the flights we wanted to book over the phone (when it was much easier on the internet), we found that the fare on the phone was also 30 euro more expensive than the internet (when queried, the operator said that the "fares on the phone are not available on the internet"). Anyhow, to cut a long story short, we were looking at 30 euro extra per person, a total of 60 euro more expensive than the internet. The voucher was worth 220.
To cap it all off, all the phone operator wanted was our email address, she asked us to send the voucher in with the email confirmation which she would send to us - i.e. we could have done it online all along! In the end they issued the refund by cheque about a month later.
So the moral is beware when redeeming those vouchers!