  The optical reflectivity of Pb$_{0.865}$Sn$_{0.135}$Se and
Pb$_{0.75}$Sn$_{0.25}$Se solid solutions was measured in the THz spectral
region energetically corresponding to bulk optical phonon excitations and in
the temperature range from 40 K to 280 K. The analysis of
Pb$_{0.75}$Sn$_{0.25}$Se data performed within the dynamic dielectric function
formalism revealed a new effect due to the electron-phonon coupling resulting
in resonant changes of LO phonon frequency for energy gap equal to zero or to
LO phonon energy. This effect is absent for Pb$_{0.865}$Sn$_{0.135}$Se that
exhibits an open energy gap with trivial band ordering at all temperatures.
These results show that reflectivity in the THz range constitute a versatile
experimental method for precise determination of band inversion in narrow-gap
topological materials. For Pb$_{0.75}$Sn$_{0.25}$Se the transition from trivial
insulator to topological crystalline insulator phase takes place at temperature
T$_0$ = (172 $\pm$ 2) K.
