This tip is for the non-US Gmail users, wanted to try out new Gmail Call features. According to Google, the latest Gmail feature is only available in US, but that's not actually true. If you set Gmail interface language to English (US), you should see "Call phone" option in the chat roster. You can also buy credit using Google Checkout.
If you're not in US and you want to add credit to your account, make sure you don't click "Upgrade your account" button from Google Voice. Because, it gets you a Google Voice number. And if, you click the button, you'll no longer be able to buy credit.
Here's what you'll see after clicking on "Upgrade your account":
Tip: You can cancel upgrade if you use a US proxy (UltraSurf).
A Google spokesperson said: "We're looking into this and will keep you updated."