I guess it depends what you want to do with them after but the previous answer is correct. In all case you need to use affinity chromatography to separate them after extraction. In our protocol for wheat histone, we used phosphorylation inhibitor in all step and separated them on nanoflow-HPLC system prior to the mass spectrometry. There is a large number of Purification kits on the market from commercial life science companies that you can have a look at if you want to have a lysate of pure phosphorylated protein for other uses than the mass spectrometry.