We have a problem, when we try to transfer an external call to another external call in blind transfer mode the call failed and we receive 403 Forbidden.
When the transfer to external user is attempted, we have that the originator is external user again and target is another external user.
We had a similar problem. The issue turned out to be that the http://www.microsoft.com/LCS/OutboundRouting application was not running on the Front End Server. Make sure this application is enabled.