Silverlight PollingDuplexHttpBinding and Cross Domain Issues

I created a silverlight component and a WCF service, both communicate using PollingDuplexHttpBinding to enable push and pull functionality.

Works well on our domain, however a host outside of our domain does not completely call a web service that is publicly exposed.

The clientaccesspolicy file is available and downloadable from the root, I even added a crossdomain. So from server A to server B (A and B in the same domain) I see send requests from silverlight client (in FireBug), between A and C (where C is external server), no calls, not even for the clientaccesspolicy file.

Is PollingDuplexHttpBinding and cross domain still available (can't find anything about it) or is there something else I can look at?