1. Check the network topology as below: U1960 is behind Firewall, and interconnected with PSTN via SIP trunk, FW act as a proxy server.
2. Capture packet at FW, from the packet, FW already forward Invite message, but PSTN respond a 403 forbidden message.
3. Check the invite message FW forwarded, we can found the From header is anonymous, which may be the reason PSTN refuse this call.
4. Capture packet at U1960 side, and found IP phone have sent an correct calling number, but U1960 sent an anonymous calling number to FW, then check the service of this specific user, and can found this user have “Calling line identification restriction” service enabled.
Disable CLIR service on U1960 and test again, the outgoing call was made successful.