IPKall and FreeDigits have the exact same business model for their free phone numbers - viz. they charge exorbitantly expensive rates to the phone company to connect calls from callers to you (which the caller themselves do not see but only the phone company does), which compensates these two providers for offering these phone numbers to you for free.
In all honesty, this sort of parasitic thing is only possible because of the distortion caused by government intervention in the phone market. It might be drawing to a close but not yet. Either the government intervention will only get worse in order to stop this 'unlawful' profiteering or the socialized conditions that exist in the first place will have to be dismantled so that the market can be reorganized on rational lines.
See this thread for info on these free Iowa numbers:
http://www.dslreports.com/forum/remark,17789735