Breach of confidentiality, how to sue?

My phone provider for the past 2 yrs has made it possible for the previous owner of my number to still log onto the Internet account they had set up. The phone company had updated all my details into the account and the person has been receiving my bills and personal details. I couldn't register online to look at my bills because it was still registered to the previous person with their password. How do I go about suing the company.