It used to be that you had to lease bridge lines by the call or by the month. If you were teaching several programs or teleclasses, or running coaching groups, this could add up.
In recent years there have been a proliferation of free bridge lines. These have become so popular, that even some of the coaching schools have begun using them, rather than leasing their own lines. They work equally well. (If you're wondering how these companies can do this, they get a few cents per caller into the lines from the phone companies.)
Here are some of the more popular ones:
www.FreeAudioConferencing.com [this is the one I use most often] allows up to 100 participants and you get both a host pin code and a participant one. Features include no need for reservations, conference mode and muting.
https://www.freeconference.com up to 100 participants. Features both reservation and reservation-less numbers. Mutable lines.
http://www.freeconferenceservice.com/ (up to 50 participants) Features include conference mode and mutable lines.