The telephone is growing up. Yes, you can now use your computer as a phone. Making a telephone call via your computer doesn't mean you have to sacrifice quality either. According to data collected over the last 12 months by Minacom's single-ended service quality test system, only 1 out of 50 calls in North America were considered to be unacceptable - 1 in 10 worldwide - while greater than 85% of VoIP (Voice Over Internet Protocol) calls exceeded the average PSTN (Public Switched Telephone Network) quality over the same period. With clear advances being made in call quality, you can be confident that the cost savings you will realize by using your computer as a phone will not be due to compromised call quality.

Essentially there are three ways to make calls using your computer:

Computer to Computer
IP (Internet Protocol) Phones
ATA's (Analog Telephone Adapters)

There are some fundamental basic things you'll need on your computer for computer phone calling to work (but most computers have these anyway):

Computer Sound Cards
Computer Microphone
Computer Speakers
28,800 bps or higher Internet access (essentially high speed internet)

You may hear another term used to describe using your computer as a telephone -Internet Telephony. Read more here for additional detail on how to use your computer as a telephone.