You can build it completely in the cloud with a service like twilio(1). This would be good to get a temporary setup going very quickly.
If you need something a little more permanent and flexible, you can easily build it yourself with asterisk(2) or freeswitch(3). If you know what you're doing, you can turn a centos server into a dial-in conference bridge in about half an hour. This will open the door to all kinds of awesome like skype bridges, web embedded soft phones and whatever custom apps you can code up. We once created a conference call where participants could drive a robot with DTMF.
(1) http://www.twilio.com/
(2) http://www.asterisk.org/
(3) http://www.freeswitch.org/
