High pressure Raman spectroscopy and x-ray diffraction measurements have been carried out on Pb _1-x Ca_xTiO₃ (x = 0.10 and 0.30). Using high pressure Raman spectroscopic data, it is observed that the phonon instability responsible for the ferroelectric phase reappears in the paraelectric phase after a critical pressure. The observed critical pressures in the Ca²⁺doped PbTiO₃ system are much lower than the unique pressures suggested for PbTiO₃ based materials. A suitable explanation is given to explain this lowering of critical pressure. It is also shown that the ferroelectric phase which stabilizes in the paraelectric phase has a tetragonal symmetry with space group I4cm