It certainly used to be possible when analog voice/fax/modem cards in a PC plugged into the voice telephone socket.
MS even had an application in windows (phone dialler IIRC) which acted as a "handset", although you still needed to connect the analog modem card to speakers and a microphone (usually via a soundcard).
I've just checked, and it's still in Win XP (pro, anyway) - dialer.exe. You should read the CHM file!
If you have an "old" analog voice/fax/modem card in your PC, and can connect it to your phone socket - NOT ADSL socket - you are good to go!
Having said all that, you might find it cheaper to go with an IP telephony solution, although depending on your phone package, standard voice calls over POTS can be very cheap, or even free.
SD