If you have Google Voice you can use the Hangouts app to place VoIP calls to the US for free.
There's also Wi-Fi Calling from hotel rooms and such with the Verizon SIM card that should allow free calls to the US (though I don't have Verizon so someone will need to confirm that this won't result in fees). This should be a simple matter of
enabling it from within the iPhone's settings. Of course, unless you have the newest iPhone, you'll have to swap SIM cards every time you want to do this.