Login to the Web User Interface of the phone.
Go to Advanced>SIP/RTP> SIP>Network Identity Port: 5060
Go to Advanced>SIP/RTP> SIP>Listen TCP Port: On
Go to Advanced>QOS/Security>Security>Filter Packets From Registrar: Off
If you previously configured an Identity on the phone you may need to disable the Identity by turning it Off.
Go to the Directory of the phone, from the Web UI, and in the number field enter the SIP URI for the far end device you are trying to call. Example: sip: 192.x.x.x where the 192.x.x.x is the actual Public or Private IP address of the other phone.
Set Number Type: SIP, Outgoing Identity: Active, and then click add.
Click “Save” and then “Reboot” at the top of the page.
Repeat Steps 1 – 9 on the other phone.
Place a test call by pressing the “Directory” key on the phone, select the IP address of the phone you configured in Step 7, and then press the dial key.
The call should complete.