Lot of people use Google Voice to Make International Calls. The main reason being it is cheap and very easy to make calls and get connected with family and close friends. But by making calls we never know when the balance on the calling credit goes to 0. To avoid that happening, Google have implemented the following:

So based on your feedback, we implemented the ability to auto-recharge your account: just go to your billing page (click on the add credit link next to your balance) and select the recharge amount. When your calling credit dips under $2 or 2€ we will automatically charge your credit card on file for the selected amount.

This feature becomes available after your first purchase and can be changed or disabled at any time.

via Gmail Blog