Voucher balance available online or by phone?

Is there a way to tell the balance of an eVoucher online or by phone?

I have an eVoucher from a cancelled trip, and I tried to use it to book a new ticket. I entered the eVoucher number when booking the ticket, and then a message appeared stating that the eVoucher was applied, and I also entered my credit card, figuring that the ticket price was more than the voucher...and the full purchase price was applied to my credit card.

Thanks.