First, notice how all their area codes are in rural areas (e.g. one I have is area code 218 – rural northern Minnesota). The FCC allows rural telecoms (e.g. Northern Valley Telecom) to charge long distance companies (e.g. AT&T) higher interchange rates. This was originally intended to be a way subsidize rural telecoms that have lower volumes and higher costs. The local telecoms split the fees with the free conference call providers (along with other service that do this like sex chat lines).
Google (Voice) is pissed about this. Good NYTimes article on the topic here. With a sane government / FCC, you’d expect these free conference calls to go away eventually.