Amy,
That would require you system to use the Trunk-Related Information > Call Routing Tables and change the Call Routing Key to Outside Party Number to take the Incoming Number and block the numbers you don't want to come in and allow all others.
Let's say you wanted to block 555-555-1234 and allow all other calls.
The first entry would be 5555551234 routed to Call Routing Announcement that hangs-up on the caller.
The second entry would be + that routes to what the call would originally go to; depending on the Trunk Group this could be static or another Call Routing Table using the Call Routing Key of Trunk Number.
The third entry would be E that routes to what the call would originally go to; depending on the Trunk Group this could be static or another Call Routing Table using the Call Routing Key of Trunk Number.
Thanks,
TE