How do I restrict the use of my VoIP gateway by PSTN callers with a PIN?

Problem:

Solution:

Under FXO tab, set
PSTN Caller Auth Method = PIN
PSTN Caller 1-8 Pin = < PIN number > (0-9 only)

When a PSTN caller calls, a Beep tone (PSTN PIN TONE) will play continuously to
prompt the caller to enter the PIN number (followed by the # key). If PIN is
wrong, the PIN tone will restart and the caller can re-enter the PIN again.
He can retry the PIN up to 3 times. If PIN digit times out or the PIN tone
expires, or 3 retries are used up, the SPA plays the reorder tone. The caller
must hang up to try again. If the PIN is accepted, the caller hears a
regular dial tone where he can dial the desired VoIP number