If your call to a free conference call service is blocked, then this number is being used by companies participating in "traffic stimulation". In a nutshell, some rural local operating carriers are able to charge overly exorbitant costs for inter-connecting carriers to send calls to because of their limited population in comparison to their generally fixed infrastructure costs. This has been further leveraged by others, such as "free" conference calling operations, where they specifically have them operate out of these exchanges (though not physically located there), and the conference calling company as well as the local exchange carrier share the profits of pumping large amounts of traffic into such high-rate exchanges. It is currently a highly political debate, and many carriers have began blocking calls to the exchanges participating. Our upstream carrier, Level (3), does not allow calls to these areas, as well as a growing number of others.
Some additional information on this topic can be found here: http://en.wikipedia.org/wiki/Traffic_pumping
Press8 unfortunately cannot send calls to these areas. I suspect though that this will change, since carriers and service providers like Level 3, Google Voice, and other telecommunication giants have already refused to continue sending traffic there, and will force new regulations on these exchange carriers participating in traffic pumping.