The iConference application allows iPhone users to make ad hoc conferences with anyone anywhere in the world.
iConference is basically a native front for the conference calling service, Ringful. When you’ve picked your contacts or entered in the numbers you want to call, you and your recipients will receive a call from the service which will connect you all in a conference call. From within the app you can also manage the call, like viewing who’s connected in the conference, muting, or removing people from the conference.
The service isn’t free, but you’re given a free $5 credit to try out the service. You’ll be able to add more money as you go.
[Source - ModMyI] Cydia