I'm not entirely sure what it is you're asking - are you dialing the back door number and not being routed to where you expect to be?
By default the routing rules are configured such that if you forward a call into Unity and it's not from the extension of a subscriber, you'll go to the opening greeting call handler (which is what I assume you mean by default call handler here). If it's a direct call and it's not from a subscriber's phone you will again be sent to the opening greeting call handler.
The attempt forward to greeting (for forwarded calls) and the attempt sign in (for direct calls) are the only two routing rules in the system that will send the call anywhere other than the opening greeting.