did not try myself yet, but seems promising:
LANDLINES AND MOBILES
Canada,China,Cyprus,Guam,Hong Kong,Malawi,Malaysia,Puerto Rico,Russia,Saipan,San Marino,Singapore,South Korea,Thailand,United States,US Vigin Islands,Vatican City
LANDLINES ONLY
Argentina,Australia,Austria,Belgium,Brazil,Bulgaria,Chile,Croatia,Czech Republic,Denmark,Estonia,Finland,France,French Antilles,Germany,Gibraltar,Greece,Guadeloupe,Hungary,Iceland,Ireland,Israel,Italy,Japan,Liechtenstein,Luxembourg,Monaco,Netherlands,
New Zealand,Norway,Panama,Peru,Poland,Portugal,Slovenia,Spain,Sweden,Switzerland,Taiwan,Turkey,United Kingdom,Venezuela,Zambia
UPD: “Free” conditions, fare enough IMO: http://support.gizmoproject.com/FAQs/freecalls.php