They can use your credit for a refare over the phone, however it has to cover the full amount of the fare difference (not possible to combine the credit with a credit card for the refare). This is because the refared ticket can only accept 2 "forms of payment", one of which is the exiting ticket value so the remaining "form of payment" needs to cover the rest of the difference. There's technically a work around for this in most cases, but it involves making a new booking and giving up your existing confirmed seat on that flight before confirming the refared seat. Given how new their agents are, the likelihood of finding an agent who knows how to work around it and do it without losing your seat entirely is questionable and don't really recommend taking that risk.

The credit, including a FFC, can also be used for premium cabin buy up offers, or seat purchases over the phone, but it does have to cover the full cost. The FFC has to be converted to a different credit to be able to be applied towards ancillaries. In this case there isn't an work around still within the policy to avoid needing it to cover the full cost....an agent can process two separate transactions (one for the credit redemption and one to charge a card for the balance due), however that is very much outside of what their policies dictate should be done