The calling party does not have the right to call the called party.
If the Call Park Directory Number (DN) is assigned to a partition, verify that the IP phone is assigned to a Calling Search Space (CSS). Reset the Call Park pattern, the devices associated with the CSS, and the partition. If there are no partitions involved, reset the pattern and phone.
For more information on CSS and partitions, refer to Understanding and Using Partitions and Calling Search Spaces With Cisco CallManager 3.0.