If your voice gateway is SIP or H323 controlled use the "reject voice translation rule":
http://www.cisco.com/c/en/us/support/docs/voice/call-routing-dial-plans/61083-voice-transla-rules.html
Here is a doc on how to achieve the same on CUCM (needed if you are using MGCP):
https://supportforums.cisco.com/document/77241/technotes-how-block-incoming-calls-cucm-8x-based-ani-mgcp-gateway