Parece que você pode fazer isso com a senha protegendo o recurso, aqui está um exemplo:
Da página:
Here's how it works - you pick up the phone and dial 555 (Channel Spy Feature Code), The system will ask for a password. If the correct password is entered spying ensues. IF the incorrect password is entered three times the call is dropped. All I did was add a couple of lines to extensions_custom.conf
;Password Protect Channel Spy
exten => 555,1,Authenticate(1234)
;Password Protect ZapBarge
exten => 888,1,Authenticate(1234)