Outbound numbers dialed from the PBX that would go to the PBX should remain local, rather than going out to the PSTN and then back in, taking up two trunk channels.
An example of this is when someone just has their DID number on their business cards/email signature, and people from inside the company call that number rather than looking up the extension. Another example is for meet me conferences, where someone sends an email to a bunch of people (both internal and external) and posts the external meet-me number (external, of course), and you get a bunch of internal people all calling that number (each taking up two trunks, which means external calls might not get through).
The module would effectively need to reverse engineer all the dial rules and route dial patterns, and come up with the PBX versions of the numbers listed in the DID (eg, if you have 9-prefix dialing, it needs to have the 9, if you allow 7/10/11 digit dialing, it needs to have an entry for each). Complicating this is when a provider sends in a 10 digit DID, but expects you to dial 11 digits - the dial rules won't match.
This should wait until after the trunks rewrite (2.3?) is done, since it will change how/where everything is stored.