I think I see what you mean. Just like dialing "9" selects a set of lines, there still needs to be a number to dial out on the line.
Here is the scenario I'm looking for with IP trunking over a WAN to multiple sites. A call that came into 911 trunk #1 at Call Center A, which rings all phones that have that line appearance, needs to be sent to Call Center B, but not to a set extension in the Call Center. Instead it needs to appear in call center B the same way it appeared in call center A, as ringing on a trunk that appears on all sets that anyone in the call center can answer. Call Center B picks up the call and receives caller ID of the transferred call. In a call center you can't expect DN 3201 to be manned all the time and Call Center A does not have the time on a 911 call to cycle through extensions trying to transfer the call to call center B.
If I had to call, for example, 3201 at site B from 2201 at site A, then that wouldn't be considered "Trunking" to me. That would be standard IP Telephony and the purchased package of "IP Trunking" would be a major deception. I would see in that scenario where I would need unique DN's at each of my sites and Proper routing codes to get to the different sites on the WAN.
So if I can not really create a set of incoming trunks at site B to appear as lines on the PBX for me to transfer to with the "IP Trunking" package ordered for the BCM's, then I need to cancel the IP trunking packages and stick with the standard telco way of transferring calls.