So the call is going out the PRI to an IVR, then coming back in on the PRI to an LDN appearance? The caller ID will be based on what is sent by the IVR, which I assume is it is not sending anything which is why you are seeing the main number. I am not sure how you expect it to show the caller's extension unless I am misunderstanding the setup, from the Mitel's perspective, this is an external caller on a PRI channel and not an internal caller.