Polar phonons can induce electric fields in an adjacent layer, whether non-polar or polar, producing remote phonon scattering of electrons. Treatment of remote phonon scattering has been based on the dielectric continuum model which takes only the electrical boundary conditions into account. We show that crystals whose polar modes satisfy both mechanical and electric boundary conditions cannot introduce remote phonon effects in the absence of dispersion. Further, even in the presence of dispersion, remote phonon effects are negligible, as a consequence of the necessity of satisfying mechanical boundary conditions.