In our organization we needed to be able to use people's extensions when the report a trouble ticket so I made the following change to the class.validator.php file starting on line 131:
function is_phone($phone) {
// check to see if the user has entered an extension beginning with an x or ext and adjust min-length accordingly
$phone_min_length=preg_match("/^(x|ext)/i",$phone) ? 4 : 7;
// remove all non-numeric characters (original implementation was prone to missing things
$stripped=preg_replace("/","",$phone);
// adjusted max length to work with 7 digit number + 4 digit extension
// NOTE: I also updated db ost_ticket.phone length to 25 to accommodate longer numbers
return (!is_numeric($stripped) || ((strlen($stripped)<$phone_min_length) || (strlen($stripped)>14)))?false;
}