I've never been able to get a phosphate reading, even when I had a patch or two of cyano, when I went through my Bryopsis phase, etc. But my tank does go through an awful lot of calcium, more than I think should be needed for my coralline algae and single open brain coral
I feed only drained mysis so the phosphate input is probably not too high, but I suspect it's being bound by the calcium before anybody else gets to it.