Sort Location Listing by Extra Field Phone Number

gistfile1.txt

// Sort by extra field. e.g "phone". 

add_filter('wpgmp_sorting','wpgmp_sorting_extra_field',3,2);
function wpgmp_sorting_extra_field($sorting,$map) {

$sorting['phone__asc__num'] ="Phone A-Z";
$sorting['phone__desc__num'] ="Phone Z-A";

return $sorting;
}