The phase delay introduced by photodetectors can be affected by intensity, reverse bias, and temperature through different effects. An optical pilot tone superimposed on the detected signal allows an independent measurement of such phase errors in the complete photodetection chain and provides an opportunity to correct them. This allows to further separate readout noise from the measurement, providing a more performant and intensity-invariant phase readout. We test the functional principle on a setup demonstrating an improved phase noise performance and a reduced phase walk below 10 mHz in particular. This benefits applications that require accurate timing or signal phase determination with photodiodes.