Tom2018 : I definitely can't do that, although that was my initial idea, I created another network element to do it, however that caused problems, it was seen as a duplicate. The IPBX already has several IP trunks, and I don't want to restrict calls on those, so I can't do it that way.
Now I'm thinking of wasting SIP trunk devices to do it, I have some laying around (and it's a temporary setup).