I'd have to hear the voice mail message to believe what you're saying. In my daily life, when one of my friends says, "That's great. Just great" it's totally clear whether he's being sarcastic or not. Because he intends to sound sarcastic, and changes his inflection.

Now in this case the message was the same, so the inflection didn't carry any content. But the subjects were told Mark was leaving a message for his friend. So it's a reasonable assumption that Mark is using an inflection that his friend will recognize as sarcasm. I would assume that unless, as in the second case, they told me he was trying not to show sarcasm.