In the gateway configuration screen of the Cisco CallManager, verify that the number of significant digits mentioned corresponds to the number of digits in your dialing plan.
If any digits are prepended using the prefix Distinguished Name (DN) option on this screen, ensure the new digit string matches the DN. If it does not match, then remove ormodify the prefixed digits and update. Reset the gateway after making the necessary changes.
If the Called IP phone is in a partition, the most common reason for receiving re-order (fast busy) is that the calling party does not have the rights to call the called party. In the case of calls from the PSTN, the Calling Search Space (CSS) that the gateway belongs to must include the partition of the called party. For more on CCS and partitions, refer to Understanding and Using Partitions and Calling Search Spaces With Cisco CallManager 3.0
If any CSS or partitions are used, restart the devices in the CSS from the Route Plan menu. Do the same for partitions, and reset the gateway.
If no CSS or partitions are used, restart the gateway and the IP phone.