What is VIOP?

good internet connection in both side, microphone, pc, and Yahoo mesengger

Actually its pretty simple. Subscribe to voip provider like Onesuite or Vonage or which ever you prefer or suit your needs and presto you can make calls right away. If you want to use your pc then you download softphone program like SJ phone for Onesuite and install it on your computer and thats it. If you want to use it like a regular phone then you need an IP phone or a ATA (Analog Telephone Adapter) device and attached it to your regular landline phone. No need to set up an asterisk or trixbox for home use unless you want to build some kind of network of voip phones in your house. Most voip providers nowadays are easy to use and user friendly.

Setting up a VOIP is a little complicated. You should have enough bandwidth to ensure voice quality. Plus the specs of your server. You can try asterisk. It has many documentations online.

There are two possible scenarios for internet telephony. Either both participants of the phone call are connected to the internet or one of the participants is using the classic voice network. In the first case, no additional connecting costs will arise besides costs for your internet provider. Therefore, flatrate users might use VoIP for free. On the other hand, if one of the users is not directly connect to the Internet, the class phone network has to be used somewhere. This means the IP network has to be left sooner or later via a so called gateway, which then handles reaching the other participant using the voice network. Costs then depend on the company providing those gateways. Here, prices should be compared and phone calls below "usual" voice-network-only costs should be easily possible.

Most all your instant messengers have something like this. Like msn messenger, but it is not as clear. I now use skype, which is free to download and the other party downloads it and you talk over the computers with a headset or mic. It is as clear as a phone conversation. You can add a webcam and actually see each other too. I use this to talk to my son who is in germany and I am in the US.

A good site to start is http://www.voip-info.org/wiki/ This site is really good! lots of information. I like asterisk. With this great piece of software you can turn a old pc into a full featured pbx (private branch exchange) If you have a old pc standing in your corner try www.trixbox.org