The Federal Communications Commission (FCC) is investigating Google Voice for blocking telephone calls from users that use Google Voice to call telephone numbers in certain rural areas.
From Google Blog: The reason we restrict calls to certain local phone carriers’ numbers is simple. Not only do they charge exorbitant termination rates for calls, but they also partner with adult sex chat lines and “free” conference calling centers to drive high volumes of traffic. This practice has been called “access stimulation” or “traffic pumping” (clearly by someone with a sense of humor). Google Voice is a free application and we want to keep it that way for all our users — which we could not afford to do if we paid these ludicrously high charges.