This probably has nothing to do with the OBi. You can confirm that by testing Vopium from a softphone.
Ringback tone (what the caller hears while the destination phone is ringing) varies from country to country. In much of the world, it's a single-frequency tone, e.g. 425 Hz, 1 second on, 3 seconds off. IMO, it is useful to hear the native tone, as that gives the caller some confidence that he is reaching the correct country.
If you have a special reason for wanting to hear US ringback, e.g. the call is being placed by a machine that recognizes it, you could use a (probably more expensive than Vopium) service that signals 'ringing' rather than 'progress' while the callee is being alerted.