If you are an international Google Voice user then beware of clicking the little “upgrade your account” button from within Google Voice. Doing so may render your ability to access the Google Voice web site obsolete.
If you’re not in the US and you want to add credit to your account, make sure that you DON’T CLICK the “Upgrade your account” button from Google Voice. This feature is only useful if you are in the US, since it lets you get a Google Voice number. Unfortunately, if you click on that button and you’re not in the US, you’ll no longer be able to buy credit.
There does not appear to be an easy way to downgrade your account once you have opted for the upgrade. The Google Operating System article does mention the upgrade may be cancelled by using a U.S. based web proxy server but that is a little cumbersome for the average non-techie internet user.
All is not totally lost however. Although the Google Voice web site itself becomes inaccessible, phone calls initiated from within Gmail still appear to function. Hopefully Google will fix this and simply hide the “upgrade your account” button from international users. Doing so will create less confusion and not cause users to inadvertently prohibit access to their own accounts.