We report far-infrared reflectivity measurements on (GaAs)m/(AlAs)n superlattices (SL's) with systematically varied layer thicknesses in the range 1 less-than-or-equal-to m,n less-than-or-equal-to 7. Taking advantage of an appropriate choice of the total SL thickness D less-than-or-equal-to 0.3-mu-m, we measure the frequencies of AlAs-like TO1 confined phonons from the peak of the reststrahlen band. The GaAs-like TO1 frequencies are obtained by fitting reflectivity spectra to the SL dielectric-response-theory model. Microscopic calculations of confined TO frequencies are performed within an ab initio scheme and successfully compared with the experimental data.