If you want to use an external contacts list, the Contact list and Contact list URI settings are mandatory. Publish presence is optional; when enabled, the phone sends out "PUBLISH" SIP messages with the phone's status, e.g. busy, etc.

1. On the phone's web interface, Identity (1-12) page > SIP tab, click the radio button on of the Contact list entry.
   
   
2. Enter the URI received from your administrator, VoIP provider or other external source in the text field of Contact list URI.
   
   
3. Optional: Click the radio button on of the Publish presence setting.
   
   
4. Click Apply and Save.