You just need to combine the two services.
So how to achieve Google Voice Call forwarding :
1. Register with Keku voip service. Create an account.
2. Add your Google Voice number in your Keku account.
3. Keku assigns a local number for each of your contact. As you need to forward your Google Voice calls, create a contact with your own number (cell phone) and choose a local access number nearby to your cell phone local area.
4. Log into your Google Voice account, go to settings, and click "Add another phone". Add the local access number you got from Keku here.
5. When anyone calls your Google Voice number, call will get forwarded to your Keku number. And this call will then forward to your cellphone.
For adding the number to your Google voice number, you need to verify it (Google Voice system will call you).
So how does international call forward work.
Suppose you are on international travel (say India) and have a new international number. Now you want that when people call your Google Voice number, the call gets forwarded to your international mobile number (India number).
Just your Keku account, go to the contact your created for Google Voice and click the edit at the top of the contact page. Just change your mobile number (previous) to your new international number.
This way whenever anyone calls you on Google Voice number, call will get forward to your international number.