I test a lot of water and have never seen or heard of phosphates masking a CYA test. I have experienced a temporary lowering of CYA when a lanthenum based phosphate remover is added to the pool in a large dose but it has not happened every time and the CYA results have returned to normal in a week or two. I have never seen this particular effect documented anywhere but it is something I have noticed in my testing and also in my own pool when I was experimenting with lanthenum based phosphate removers.