3CX Phone System includes the abilitity make and receive Direct SIP Calls. 3CX Phone System can receive Direct SIP Calls ( user1 @ mydomain.com ) as well as place Direct SIP calls to URIs from another domain. Please note that this feature requires setting up a DNS SRV record for your domain to receive inbound Direct SIP Calls.
The option only "Allow calls to external SIP URIs" and your "Local SIP domain". There is not found any IP range in need to set.
The direct SIP seems works if your 3CX box direct connected to the internet.