International Gmail users: Don't Click this Button

After enabling the "Call phone" feature that enables calling from Gmail to phones, Google has issued a warning for international Gmail users keen on using this new feature.
In a blog post titled 'Don't Click this Button in Google Voice', Google has warned users to not click on the "Upgrade your account" button if you are not in the US.

Even if you have enabled Gmail's interface language to English (US) - which is necessary to avail of the 'Call Option' feature - you won't be able to buy the credit required to call countries other than the US and Canada (which have free calling)

Google has stated: "We're looking into this and will keep you updated."