the format is very explicit.
It should be:
“hidden”
that will trigger specific dialplan which will execute this statement:
exten => s,n(hidecid),ExecIf($["${CALLERID(name)}"="hidden"],SetCallerPres,prohib_passed_screen)assuming your provider honors the signalling, this is what tells them not to display the number while still passing it to them.
Some provider will require a setting of “sendrpid=yes” but you would have to find that out from your provider.