The Host Not Found error message is received when you press the corporate directory on some 7945 and 9971 IP Phone models.
This issue can occur if you have changed the domain for the CUCM Publisher and Subscriber. In order to resolve this issue, you need to delete the ITL file from the phone. Complete either of these steps in order to accomplish the task:
On 79XX IP Phone models, navigate to Settings > Security > Trust List > ITL File > **# (to unlock the settings) > Erase.
On 89XX/99XX Sip IP Phone models, go to Settings > Administrator Settings > Reset Settings > Security Settings.