How to get specific table by current user login

Note that you may need to use WPDB::prepare() to prevent against SQL injections. Although, it might be arguable in your specific case…

// If the query does not work, ensure the table name is correct...
$table = $wpdb->prefix . 'SaveContactForm7_1';

$reservations = $wpdb->get_results(
    $wpdb->prepare(
        "SELECT * FROM $table WHERE user = %s",
        $username
    )
);

See WPDB class reference. Hope this answers to your question.