I'll assume you're talking PSTN calls because internal calls are all CSS/partitions.
NO, you need to do this in an H323 GW, impossible with MGCP
Reject Calls
http://www.cisco.com/en/US/partner/tech/tk652/tk90/technologies_tech_note09186a0080325e8e.shtml#reject
Call Blocking Configuration Examples
http://www.cisco.com/en/US/customer/tech/tk652/tk90/technologies_configuration_example09186a00803f818a.shtml#con11
PLEASE SEARCH NETPRO FOR ANSWERS, this has been asked PLENTY of times before
HTH
java
If this helps, please rate
www.cisco.com/go/pdihelpdesk
HTH
java
if this helps, please rate