Mashable,
It seems to be account-specific since calls in general are working. We just successfully tested calling a UK seller to make sure it wasn't an international problem.
We're seeing the instances where your buyers failed, though, and it seems to be a telco carrier issue. Not on our end, but sorry about that anyway. We're checking out with them exactly what happened in those instances to ensure it doesn't happen again.
I'll also try calling you through Ether tomorrow if you're available (it's a bit late for you tonight).
Scott