limiting extension privileges in trixbox

[Elliott Jeyaseelan]

Hello,
 
I am trying to setup an extension which can only make local outbound calls
thru zap channel.
 
Is there anyway to disable incoming calls to this extension from any of the
other configured extensions in trixbox?
 
If this is of any help, I am trying to setup a sip extension....
 
Appreciate any suggestions or ideason how to do this.
lliott