^^^ Thx all.
^^ I think I found the area, what to do and how to fill the balance into your Google voice account.
1. go to
www.google.com/voice and login with your Google account
2. In the left column, at the bottom, in green color where it says,... your credit is 0.00, there is a "+" sign. Click that.
3. it will bring a "10.00" credit button. Press that and put deposit by giving your credit card number.
4. For India, it charges 2 cents per minute, so, now you have credit in your account to make a call to India.
I think, while looking on this site,
https://support.google.com/voice/answer/141923?hl=en&ref_topic=1707988in the right column " buy the google voice credit " button-link is in grey color and that tells me, they are in verge of not taking more credits? Perhaps they are closing the GV ( May 15 effects ) and that's why