Watch this firs to know what is Google Voice.
Each step of Google will be like a plan of a product for at-least for 10 yrs how it should be. Each product is designed considering our future use. If we are in US and now we are open for Google Voice account without any invitation. Early it was like someone should invite you as like in ancient period when we create Gmail account. Now it's like open to all.
Google is taking necessary steps to implement this for the users outside US. But we shall hope that should be done ASAP.