Having the PBX gurus remove the DID number for incoming
lines that are not approved might not be the best. This limits
the number of inbound trunck to the PBX. The advent of
caller ID and ANI numbers allows most folks to determine
what their line is.
Changing the class of service on the extension to only
allow station to station calling, deny call forwarding,
etc, is probably easier. Many folks still order 1MB
lines to bypass the PBX cause the PBX causes interference
so that fix doesn't work. Then you have to get the
telco to make the telephone not accept incoming calls.
Policy backed up by effective enforcement seems to make
an excellent solution.