Conference calling is generally an over priced service in which multiple parties are connected in one conference with audio and sometimes video.
There are many companies that offer this service but sometimes you want to have your conference for free (the angrybyte style!).
To keep things free you will need a good Internet connection on every site with conference parties, then you need to choose a software to make it happened.
For now let me suggest oovoo
I don’t have anything to do with oovoo but I have to tell you it is an elegant software, you can video conference for free,and see multiple users at the same time. The audio quality is good and you can even make some free phone. calls inside the US with each new account.