BILLED EXTRA AFTER CONTRACT SIGNED

It has just come to my attention now that the debit order has went off my account, that CELL C has charged almost 10% extra on each contract that I have with them. They have stated that an sms was sent to the 2 cell c numbers that I have, informing me of the increase of each contract. This is absolutely pathetic and alarming considering that a service provider can debit any amount they want out of our accounts without proper permission. At the same time, I do not get any statements or invoices from cell c, not emailed or in the post. This is also a problem, because the increase could have been communicated on the invoice or statement. I don't agree that the increase should be implemented on my account, as I have not authorised it. As well as, if I signed a contract for 24 months for a certain amount, that's what I should be paying for 24 months. I have had many many contracts in my lifetime, and this is the first time I have experienced an increase DURING the contract. I do not agree with this.