If your provider raises the tariffs applicable to your contract, you have the right to terminate that contract free of charge.
That is also the case when you have a fixed-term contract, which would normally imply a cancellation fee. This cancellation free of charge remains possible during the 3 months after your operator informed you of his intent to raise the tariffs.
Caution: the right to terminate a contract free of charge does not apply when the increase is the precise result of an increase of the consumer price index (and if this indexation is stated in your contract).
Even when your operator changes other provisions in your contract, he has to notify you at least one month in advance. In that case as well you can terminate the contract free of charge.
For fixed-term contracts the cancellation remains free of charge for three months as well: see above.
Caution: the right to terminate a contract free of charge does not apply if the modification of the contract is the result of a legislative amendment or if all modifications are exclusively and demonstrably to your advantage.
Important as well: if you have a device that was part of a conditional sale and that you are paying off via your telecom subscription, the residual value according to the repayment table, remains due. This applies to all forms of cancellation, and therefore in the case of a permanent contract as well.