As far as I know, BoltzTrap is for electronic transport and while it does capture electron-phonon interactions at a rudimentary level via a relaxation time, that does not imply it computes phonon transport. Phononic thermal conductivity is limited largely by anharmonic phonon-phonon scattering rather than phonon-electron processes (except at very high carrier concentrations) so you need a different code for it. Some examples are shengBTE and AlmaBTE, which solve the phonon Boltzmann transport equation iteratively while using dispersion and phonon coupling from first principles. In some general sense, they are the phonon counterpart of BoltzTrap, albeit with a more extensive BTE solver in place of the RTA.

Thank you all for your quick and helpful response.

BoltzTrap code relies on fixed relaxation time approximation. To know how to calculate actual relaxation time, you need to go through literature beyond this code.