Yes, you need to call the agent to see how much you can get back/ if you can get anything back.
$500 is a pretty fantastic deal from the US to the Philippines as it's a long way. Any reason you don't want to go?
If you can't get a refund, you may as well go and get a low cost flight to KL/ Bangkok/ somewhere and explore a bit! That would be much better than wasting money of the flight ticket.