IPKall was a public switched telephone network to voice over IP call forwarding service.
Users were able to register with the service to obtain a phone number chosen from several Washington State area codes and have all calls to that number forwarded to their Session Initiation Protocol or Inter-Asterisk eXchange uniform resource identifier, including an Asterisk server.
IPKall's popularity stemmed from the fact that it was probably the last remaining, widely accessible US-based VoIP service offering free-of-charge PSTN numbers.
The IPKall business model appears to be based on its ability to charge termination rates for calls reaching its subscribers, estimated at about $.01/min, as the service is free for end users.
[1] The wholesale price for major interexchange carriers to terminate calls to independent telcos in rural Washington (state) and similar locations (such as northern Minnesota and Iowa) is often several cents a minute higher than typical US48 rates; this money is effectively a subsidy to the destination local rural exchange.