The first- and second-order Raman scattering and IR reflection have been studied for hexagonal InN layers grown on (0001) and $\text{(11̄02)}$ sapphire substrates. All six Raman-active optical phonons were observed and assigned: $E_2(\mathrm{low})$ at $\text{87\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ $E_2(\mathrm{high})$ at $\text{488\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ $A_1(\mathrm{TO})$ at $\text{447\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ $E_1(\mathrm{TO})$ at $\text{476\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ $A_1(\mathrm{LO})$ at $\text{586\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}},$ and $E_1(\mathrm{LO})$ at $\text{593\hspace{0.05em}}{\mathrm{cm}}^{\mathrm{-1}}.$ The ratio between the InN static dielectric constants for the ordinary and extraordinary directions was found to be ${\epsilon }_{\text{⊥0}}{\mathrm{/\epsilon }}_{\text{∥0}}\text{=0.91.}$ The phonon dispersion curves, phonon density-of-state function, and lattice specific heat were calculated. The Debye temperature at 0 K for hexagonal InN was estimated to be 370 K.