Chinese cell phone used in US

While my friend visits the U.S. with her phone that she brought from CN, which also has the SIM card of her carrier in CN, a call she makes while in the U.S. to a U.S. phone will actually be routed, via roaming, to CN first, and then back to the U.S.? This sounds expensive. What if she brings her phone here, and gets a SIM card from a U.S. carrier? Will a phone purchased in CN be compatible with SIM cards from U.S. cell providers?