There only 2 mobile phone service provider, China Mobile(90%market share) and China Unicom(10% market share).
China Mobile phone number usually start with 134,135,136,137,138,139
After 2007, there 3 new range added 150, 158,159,

China Unicom number start with130,131,132,133
After 2007, new range is156.

The Costs:
Dial from china to USA(America) directly: the fee will be 0.3/minutes+0.8/ 6 second= 8.3RMB/minutes, when travel to another province in china than the SIM card belong to, add another 0.2RMB/minutes.

Add 17951 prefix:
the rate is 0.3+2.4=2.7RMB/minutes, when travel to another province in China, add another 0.2RMB/minutes.

Add 12593 prefix:(this is only useful when you stay in the province the SIM card belong to.) , the rate is 0.3+0.7=1 RMB/minutes, when you travel to another province in China, the fee will be same as dial directly:8.3RMB/minutes.

So the best way to reduce cost is add 12593 when you stay in the area the sim card belong to & add 17951 when you travel to another area in China.

While China Mobile SIM card travel to USA(America), the fee will be:
Call china: 12.99RMB/minutes
Call US: 5RMB/minutes
Send sms: 1.99RMB/each
Receive sms: free