(Since this solution depends on reliable Internet service in all locations involved, please make sure that you have good service in each location where you put VoIP extensions and that that service is not saturated, or over used.)
Making international calls to and from the U.S. incurs per minute charges in either direction, whether you call your offshore office or whether your offshore office calls your U.S. office or any other U.S. number.
You can solve this problem by putting VoIP extensions with U.S. phone numbers in your international locations. This allows you to call between your U.S. office and international offices for free. It also allows you to call U.S. all numbers from the international offices for free, provided your VoIP service is flat rate.
Find the Right Provider
Not all VoIP providers support this scenario. Here are links to two that I know of, one on the West Coast and one on the East Coast:
Here is a link to a good solution for residential customers who call offshore often:
Bill Harrel – www.williamharrel.com