If you have not paid your bill the operator has to warn you before discontinuing the services.
The first written reminder is free of charge. The fees for subsequent written reminders must not exceed 10 euro.
Mind you: a text message sent by your operator as a reminder, can also count as a written reminder.
In addition, the operator may charge you penalty interests on the delayed payment. That interest must not exceed the legal interest.
Before completely stopping the service the telephone service operator has to provide a restricted service to you without charging extra fees.
The limited service is an intermediary step. The operator must keep it for as long as necessary to ensure immediate payment.
The operator cannot set up the restricted service if you argue a fee. A condition is that you pay the uncontested part in due time.
In order to restore a normal service after an interruption of the services, the operator may charge you a maximum of 30 euros.
Finally, in case of fraud or persistent non-payment (e.g. second non-payment after a previous interruption of the service in the same year), the operator can stop all services immediately.