I must confess that I've not seen the point about rigging up a PC as a software-driven PBX using Asterisk software. Until now.
It seems that the Gizmo5 (aka Sipphone) service has peering arrangements in place with a stack of VOIP networks around the world.
And, according to IP PBX portal Nerd Vittles (what a name! -Ed) there is a technique for getting free calls on a number of the world's phone networks.
Addressable networks reportedly include Alltel, Cablevision Lightpath, Cincinnati Bell Wireless, Comcast, Cox, RCN, Sprint PCS, T-Mobile and Verizon Wireless to mention but a few.
The technique hinges on a technology known as backdoor dialling - you dial 0101 and then the 10-digit number of your choice to see if a free call will connect.
Be warned, it's teckie, but the techniques works.
Allegedly your honour...