In fact you can check any of the numbers you call are free calls by checking with Gizmo5's back door. Nerd Vittles also explains about the carriers who are using the VoIP for their call terminations. I just checked my sprint number and it is free through Gizmo5 back door dialing. Back door dialing is easily accomplished by adding 0101 to the destination number, Just dial 0101 + 10 digit US telephone number.
While the technicalities are too long for a referring article, you can read all at leisure at Nerd Vittles. In addition, the article also tells you how to add Free Calls to 40 Million Asterisk Servers with e164.org. I am proud to say that four of those Asterisk servers are mine and there are about 100-150 that I helped to implement. You might also want to read ASTGizmo to setup Gizmo5 and Asterisk together.
So head over to; Free Asterisk Calls to Zillions of Phones with ENUM and Gizmo5’s Backdoor Dialing and start making those free calls.