3CX does not seem to handle connecting to sip extensions on other domains. e.g. if you try to call the Sydney weather service on sip:+firstname.lastname@example.org using a softphone, 3cx will try to connect to the SIP extension using one of the gateways or providers. There is no outbound rule type to just connect directly with the other IP PBX / SIP extension. This obviously only happens if the softphone uses 3cx as an outbound proxy.