Coherent excitation of phonons by optical waves, one of the most important channels for light-matter interactions, provides a promising route for optical manipulation of microscopic acoustic phonons for quantum opto-mechanic and phononic devices. Prior research, such as stimulated Brillouin scattering (SBS) in fibers, mainly emphasized phonon amplitude modulation; however, coherent phase control of these phonons has not yet been well explored. Here we experimentally demonstrate a new mechanism to coherently control acoustic phonon phases by a seeded SBS scheme in an optical fiber. Interference between acoustic phonons enables either nearly total transmission or enhanced reflection of optical waves, effectively controlled by phase modulation. This new technique addresses the crucial problem of phase-controlled phonon generation, paving the way for important applications in quantum opto-mechanic and phononic devices.