Eliashberg function of RbBi2, calculated without SOC, is the highest at low frequencies (up to 1.5 THz) due to the presence of softened acoustic modes and low-frequency optical modes. When SOC is included, the Eliashberg function becomes similar to phonon DOS, which means that electron-phonon coupling only weakly depends on phonon frequency. The largest contribution to Eliashberg function comes from highly-degenerate and partially flat Bi modes in the narrow frequency range.