The dielectric behavior of poly(methylphenylsiloxane) (PMPS) was revisited to investigate the well-resolved alpha'- and beta-relaxations found on the low- and high-frequency flanks of the segmental (alpha) relaxation, respectively. Comparative studies of the alpha'-, alpha-, and beta-relaxations in two PMPS samples of widely different molecular weights and a phenylmethylsiloxane-dimethylsiloxane (PMPS-PDMS) copolymer were carried out with a focus on the dynamics of the alpha'-, alpha-, and beta-relaxations, including the frequency dispersions and relaxation times, tau(alpha'), tau(alpha), and tau(beta). At temperatures above T-g, tau(alpha), and tau(alpha') are molecular-weight-dependent and tau(alpha') has a weaker temperature dependence than tau(alpha). The alpha'-mode is identified as the sub-Rouse relaxation by its time and length scales that are intermediate between the segmental motions and the Rouse modes. There are several novel findings in the alpha'-, alpha-, and beta-relaxations. For the two PMPS homopolymers and the PMPS-PDMS copolymer, tau(beta)(T) are practically the same in the glassy states and in agreement with tau(alpha)(T) reported for PMPS confined in nanometer spaces. Both the separations between tau(alpha') and tau(alpha) and between tau(alpha) and tau(beta) increase with the molecular weight of PMPS. By applying pressure, we find that the beta-relaxation is sensitive to pressure, and the ratios tau(alpha)(T,P)/tau(beta)(T,P) as well as the frequency dispersion of the alpha-relaxation at constant tau(alpha)(T,P) are effectually invariant to pressure and temperature changes.