How To Stop CallManager (CUCM) 7, 8, 9, 10.5 with MGCP / H.323 Voice Gateway From Rejecting Anonymous (Hidden Caller-ID) Calls

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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:

Jan 30 07:42:16.892: ISDN Se0/1/0:15 Q931: RX <- SETUP pd = 8 callref = 0x1075

Sending Complete

Bearer Capability i = 0x8090A3

Standard = CCITT

Transfer Capability = Speech

Transfer Mode = Circuit

Transfer Rate = 64 kbit/s

Channel ID i = 0xA98381

Exclusive, Channel 1

Calling Party Number i = 0x0180, 'anonymous'

Plan:ISDN, Type:Unknown

Called Party Number i = 0x81, '0298889994'

Plan:ISDN, Type:Unknown

Jan 30 07:42:16.900: ISDN Se0/1/0:15 Q931: TX -> CALL_PROC pd = 8 callref = 0x9075

Channel ID i = 0xA98381

Exclusive, Channel 1

Jan 30 07:42:16.904: ISDN Se0/1/0:15 Q931: TX -> DISCONNECT pd = 8 callref = 0x9075

Cause i = 0x8095 - Call rejected

Jan 30 07:42:16.912: ISDN Se0/1/0:15 Q931: RX <- RELEASE pd = 8 callref = 0x1075

Jan 30 07:42:16.944: ISDN Se0/1/0:15 Q931: TX -> RELEASE_COMP pd = 8 callref = 0x9075

The problem becomes more difficult to solve when the Voice Gateway is configured to use MGCP (Media Gateway Control Protocol) as the control protocol with CUCM. With MGCP, there is no control in manipulating the Calling Party Number (as opposed to H.323). Despite the fallback, most engineers use MGCP as it dramatically simplifies the configuration on both CUCM and the Voice Gateway.

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: