Doesn't surprise me at all. Flights into KUL will normally be priced up in the currency of the outbound country (or USD). Flights out of KUL will be priced up in MYR. I think if your voucher was denominated in USD etc you could probably have used it to buy an MYR priced flight, but clearly it doesn't work the other way.

Or, are the vouchers, as they were initially set up last year, still only usable through a call centre?

Might be worth 'phoning to find out, but bear in mind call centres don't generally have access to online prices, so you may be able to use the voucher, but won't get the best prices.