yes, you first need to sign up for a cell phone plan to make calls in US in order to use italkfreedom. You need to use your cell phone plan's minutes even you are calling to China because you have to dial a special US number first. So you are technically making a US domestic call.
I have heard that skype phone allow you to have both unlimited calling in US and China, but I do not have personal experience with it.
If you don't call China that often you don't need the 15 dollars monthly plan. Instead, you can just buy a calling call which is about 20 dollars for 1500 minutes. Or you can always use QQ, skypye to talk for free with the ppl in China.