A high bill doesn't always mean there's a mistake: the period may have been longer, an estimated reading may have been used, or the tariff may have changed. That's why there are three checks before objecting: the reading date range, the difference between readings, and the tariff.
The objection has to be made in writing and submitted within the deadline. Because a verbal complaint isn't put on record, it usually gets no result. While the objection is being handled, keep track of the payment due date as well.