Loss of a Single Telephone CO Code

I am the PBX admin for the company I work for, we have been recently experiencing loss of DID’s only for a single telephone central office code (397). Full phone number is like: 303-397-XXXX. Does anyone have experience with resolving the cause of such a problem as this with their PBX system. We have an NEAX2400IMS with 4 T1 channels, and other backups and redundancies. Please let me know if additional information is needed to clarify problem.