As long as both your relatives in China and you (in US) are using the same service provider, you can call each other free.
For example, you can sign up a free account with CallCentric, which allows free member to member call. It doesn't matter where is your physical location, which could be in USA, China, or Europe, etc.
Certainly you can also do that without ATA. However, you would need a soft phone on PC to make or receive calls in this case.