|Depends on what type of telephone you mean.|
If you want to make calls through a normal telephone then you need an analogue voice modem and software to control it. Microsoft Dialler will do it and this comes standard with all versions of Windows since Windows 98.
If you want to make calls over the Internet using VoIP then you need a VoIP capable Modem and a VoIP service provider.
Or you can use a messaging service Like Skype or Yahoo messaging. This is the cheapest option is probably most suitable for home users. You have to pay extra if you want to make call to ordinary telephone numbers.
Then there is Mobile Express. This allows you to make call and send messages on your mobile smart-phone from the PC and it is free.