We study the lattice statics and dynamics of Si near the beta-tin -> Imma -> sh phase transitions. The pressure at and the order of the phase transitions can be determined precisely from the dynamical rather than the static properties. We also present an interpretation of measured Raman frequencies of a high-pressure structure of silicon correcting a previous assignment to the beta-tin phase, since the Imma phase was unknown. With the new assignment, the ab initio phonon frequencies display an excellent agreement with the experimental data. The sh -> Imma transition is accompanied by soft modes which should be accessible in future experiments. The phonon-dispersion curves and density of states point at irregularities of the behavior of the superconducting temperature as a function of the pressure for these phases.