Multimedia PCs do have speakers and a microphone jack, but talking into a microphone and listening over stereo speakers is not a convenient or quality way to make a phone call. You can do just about anything. There are other practical considerations. A cordless phone allows them that mobility. So mouse-moving has a high priority.
Telecommunications has a low priority.
Recently, a staff member complained that a file transfer over a modem line was taking much too long and was error-prone. Upon investigation, we discovered that her 166Mz Pentium running Windows 98 cannot transfer a 700Kb file reliably if the user is also using MS-Access. Once she stopped using MS-Office applications for the duration of the file transfer, the transfers were quick and reliable. Perhaps this was because her 166Mz Pentium was not strong enough to do both these jobs. The proximal cause was the scheduling priority.