Here we discuss a phenomenological approach for the understanding of the anomalous point defect formation in ionic crystals prior to the phase transition as well as the prediction of the phase transition temperatures. Quantitative agreement was achieved using a simple cube root law for the defect-defect interaction potential. This approach was applied to materials (AgCl, AgBr, AgI, PbF2) with different structures (rock salt, fluorite, wurtzite), different types of disorder (Frenkel and anti-Frenkel) and different types of phase transition (solid-liquid and solid-solid, first and second order). The computed defect-defect interaction leads to a quantitative description of the conductivity anomalies and to the prediction of a phase instability at a temperature which is very close to the actual transition point.