They're not closed services - they all offer out-dialling to the PSTN, most for free to various locations, if you keep a tenner (or 10 euros) in the account.
Scroll down to the bottom and look for the USA - most of the boxes are green/0 ...
Eg. Voipcheap.com will give you free calls without even paying them anything, but they cut you off after 5 minutes. (so you just re-dial) (or they did when I played with it some time back - made a half hour call to my brother in germany through it, re-dialling every 5 minutes!)
I wish I knew how they did it ...
Gordon