When a SIP INVITE comes in, the PBX needs to make a decision if that call comes from a trunk or an extension. The outbound proxy of the trunk is used for that - if it is missing then it is wide open and matches any incoming request. The PBX later assigns the call to an extension if the authentication is okay, but until then it is treated like a trunk call.
So it is easy, just set the outbound proxy and give it a try.