Phoneline PIN

If a PIN is in effect for a phone line, the caller must enter the PIN number correctly in 3 tries or less. Failing to enter the PIN number in 3 tries or less will cause calls from the calling number to be automatically blocked from calling the called number for some reasonable amount of time (perhaps 10 minutes). Further failures can eventually lead to a permanent block in order to prevent "hacking". This is done in the phone company switching equipment which has access to ANI and is not susceptible to Caller ID spoofing.