Computer to ComputerThis mode is the most common, as it is so easy and free. You need to have a computer connected to the Internet, with the necessary hardware to speak and listen (either a headset or speakers and a microphone). You can install voice communication software like Skype and you are ready to talk.
Obviously, this mode will work only if you have a correspondent who is using a computer equipped like yours to communicate. She should be connected at the same time. Its like chatting, but with voice.
This can happen not only on the Internet, but on a Local Area Network (LAN) as well. The network should be IP-enabled, i.e. the Internet Protocol (IP) should be running and controlling packet transfer on your network. This way, you can communicate with another person on the same network.
Whether you are communicating over the Internet or a LAN, you need to have adequate bandwidth. If you have around 50 kbps, it will work, but you wont have great quality. For good quality voice, get at least 100 kbps for a conversation.
Phone to PhoneThis mode is very handy, but is not as simple and cheap to set up as the other two. It implies using a phone set on each end to communicate. Thus you can use VoIP and take advantages of its low cost by using a phone set and speak to another person using a phone set as well. There are two ways in which you can use phones to make VoIP calls:
Using IP Phones:
An IP Phone looks just like a normal phone. The difference is that instead of working on the normal PSTN network, it is connected to a gateway or router, a device which, simply said, does the necessary mechanisms to get the VoIP communication running. The IP phone therefore does not connect to the RJ-11 socket. Instead, it uses the RJ-45 plug, which is the one we use for wired LANs. If you want to have an idea of what a RJ-11 plug is, have a look at your normal phone or your dial-up modem. It is the plug that connects the wire to the phone or modem. The RJ-45 plug is similar, but bigger.
You can of course use wireless technologies like Wi-Fi to connect to a network. In this case, you can either be using a USB or RJ-45 for connection.
Using an ATA:
ATA is short for Analog Telephone Adapter. It is a device that allows you to connect a standard PSTN phone to your computer or directly to the Internet. The ATA converts voice from your normal phone and converts it to digital data ready to be sent over a network or the Internet.
If you register for VoIP service, it is common to have an ATA bundled along in the service package, which you can return once you terminate the package. For example, you get an ATA in a package with Vonage and AT&Ts CallVantage. You only have to plug the ATA to your computer or and phone line, install the necessary software, and you are ready to use your phone for VoIP.
Phone to Computer and vice-versaNow that you understand how you can use your computer, normal phones and IP phones to make VoIP calls, it is easy to figure out that you can call a person using a PSTN phone from your computer. You can also use your PSTN phone to call someone on his computer.
You can also have a mixture of VoIP users, using phones and computers to communicate over the same network. The hardware and software are heavier in this case.
The images of Figures 1, 2 and 3 illustrate examples for each of the three types of connection.