I have dual band cell phone here in philippines ...

A dual band cell phone should work in the United States.

It has to do with your carrier. If they say yes, you will have to pay "Roming" charges which as you may know is paying anothr "carrier" for their " air waves " to carry your signal.
p.s. Did you know that you will technicaly be using the signal FROM the Philippines , " carried " to the U.S.\Virgina [the " roming " charges]

how caN i get free cellphone?

it depends on your phone's definition of dual band. usually the dual band phones in the phils use 800 or 900/1800. usually what is used in the states is 800 or 850/1900. if your phone supports any of the frequencies used by carriers in virginia then you're ok. but to be sure, you are better off using a triband phone so you can use it in the PI and in the US, best to use quad band phone for it to be used anywhere else in the world.