Cisco Unified CallManager (CUCM) and its Voice Gateway relies on the telecommunication provider (telco) to send the correct call details for every incoming call, to allow the system to correctly process it and route it.
One problem many engineers stumble upon is the routing of incoming calls which have their caller-id blocked. In these cases, quite a few telcos send Anonymous instead of N/A as the Calling Party Number (the number that is calling us), instead of the typical N/A string:
By default, all CUCM versions from version 6 and above will automatically reject calls when Calling Party Number set to Anonymous, making it impossible for callers with hidden ID to successfully call the company.
Solutions to Stop CUCM From Rejecting Anonymous Caller-IDs
One solution is to request the Telco to replace the Anonymous Calling Party Number with a specific numeric string. Possibilities of this happening, are quite slim.
Another solution is to convert your MGCP Voice Gateway to H.323. This will allow the usage of translation patterns for all incoming calls, and manually changing Anonymous to whatever is required to ensure the call is not rejected.
The final solution is to dive into each Directory Number (DN) and un-tick the Reject Anonymous Calls option, under the Directory Number Settings section. The Reject Anonymous Calls feature is enabled by default and will cause CUCM to reject all anonymous incoming calls:
When done, simply click Save and your done!
Simple - Fast and Effective!