There are 2 free pbx systems. These are in 2 different organizations that work closely together. They both have SIP trunks to an external providers and are billed for external calls separate.
Is there anything that can be done (on the Organisation 2 PBX) so Organisation 1 can’t make an outbound route that will send external calls to the SIP trunk belonging to Organisation 2?
If I understand what you are asking, as inbound calls over the peer trunk arrive at context “from-trunk” you can structure an inbound route to handle them. That way a remote caller should not have any more privilege than any other call arriving from the pstn.
I agree that you should not peer trunk to a PBX you can’t trust.