Under the revised Regulations, if you are a public electronic communications service provider and if, despite the security measures you take, a personal data breach occurs, you need to keep a log of these breaches and notify the ICO. You may also need to notify your customers.
A personal data breach means "a breach of security leading the accidental or unlawful destruction, loss, alteration, unauthorised disclosure of, or access to, personal data transmitted, stored or otherwise processed in connection with the provisions of a public electronic communications service."