Then that may be your issue.
The NAT page and settings determine how the UCM will formulate the messaging between devices. You should populate the page with your static public IP or FQDN, Check the SDP box and then enter the local LAN segments further down the page.
If the UCM sees an IP making a request it will use the settings to determine if the device is local or external. If local, it will then tell the device to use its local IP due to the request coming from an IP range that you entered for the LAN(s). If it sees a request coming from any other IP than the local LAN range, then it will tell the device to use the public IP.