You can use MS netmeeting which is probably on your PC already.
You wil also need a mic. The better option, to cut down background noise, is a combined mic/headphone set. If you are calling an ordinary landline at the other end you will need to pay the 'local' call charges. If it is solely a PC to PC set up then no other charges apply. If you have a 'full duplex' sound card, which is likely on a later PC then you can talk and hear at he same time.