I'll try, but someone correct me if I'm wrong.
Google Voice works like a phone operator. It catches all phone calls to numbers in your Google Voice account, and then redirects to a main phone line. In this case, the main phone line will be your cellphone line. So if your phone is on roam, then you will still be charged any applicable roaming fees.
That is a simplification of what Google Voice do. Hopefully that clears it up a bit.