There are lots of ways to achieve conference calls, some using VoIP, some not.
You could setup and Asterisk or Freeswitch server, both of which can host your conferences. Of course you then need to provide bandwidth and possibly PSTN connectivity.
You could use the conference server at
www.freeworldialup.com. They're not free anymore but at $30/yr they are cheap and have a new conference service.
You could use something like
www.Talkshoe.com to host your conference. That's available by both PSTN and SIP, but your conference would never be truly private.
You could pay for someone like Junction Networks to give you a private conference bridge. It's $20/mo for up to 10 people, accessible by SIP URI or PSTN number.
Finally, there's always
www.freeconference.com, which I used for a long time. It's hard to beat free. Well, you have to pay for the long distance call to their location in Iowa. But the service is free and the web interface is nice.
Michael