Hallo yubaille,
it is not sure, that it will help! You could block incoming calls too! Important ist traf=xxyyyyzz and here only yyyy. So check nadap and find in the tcd-categorie an unused amount (maybe 12). If you have a dialplan for example 100&&499 Then make nacds:cdcat=12,num=1&2&3&4; So the extension can dial internally. Then make a new category with exccs:cat=,traf=001212yy,....; Then give the extension the new cat with kscac/excac.
MB