Recently implemented an osTicket install. My boss wanted the ability to enter phone numbers as 4-digit (since most internal numbers are dialed that way). Simple fix.
in /include/class.validator.php there is a function called is_phone. Place the following code there and it will allow a minimum of 3 digits and a max of 13 to be entered as the phone number.
function is_phone($phone) {
$stripped=eregi_replace("(\(|\)|\-|\+)","",ereg_replace("(+)","",$phone));
return (!is_numeric($stripped) || ((strlen($stripped)<3) || (strlen($stripped)>13)))?false;
}