I could be mistaken, but I think it's pretty much impossible to get an accurate phosphate test of your tank water. It seems that I've read that the phosphates in our tanks get bound up in the substrate and rock and because of this, we can't get a proper test reading. I know several reputable LFS that don't even sell Salifert Phosphate test kits because of this.
On the other hand, you can test phosphates of your tap or RO/DI water. I'd start by testing that. That'll give you a starting point and might point to a problem with your RO/DI unit.
A couple things that can help is getting rid of detritus. When doing a water change, use a turkey baster or a power head to blow all the loose stuff off of your rockwork and suck it up. Also try getting all the detritus off of the sand that you're able to.
It's also a common idea that crushed coral can cause more of a phosphate and nitrate problem than sand. Don't know what you have for a substrate though.
Adding Chaeto to your sump should start lowering your nitrate levels soon. If possible, have flow in your sump to keep your ball of Chaeto continuously rolling.