I get the following error while fixing my wordpress plugin standards.

 Use placeholders and $wpdb->prepare(); found $sql

Below is my code...

public function get_db_value() {
    //getting meta_value from wp_usermeta
    global $wpdb;
    $billing_phone = isset($_POST['billing_phone']);
    $sql           = "
    SELECT  user_id,meta_key, meta_value
    FROM {$wpdb->usermeta} 
    WHERE ({$wpdb->usermeta}.meta_key = 'billing_phone'";
    $ansatte       = $wpdb->prepare($wpdb->get_results( $sql ));
    $users         = array();
    foreach ( $ansatte as $a ) {
        $users[ $a->user_id ][ $a->meta_key ] = $a->meta_value;
    }
    foreach ( $users as $u ) {
        $nums[] = $u['billing_phone']; //Saving phone numbers in $nums variable

    }
    return $nums;
}