E-mail contact? Problem with a cancelled contract, infoscore letter

Good day. I am having problems with an O2 contract that I have cancelled 2 years ago. I am recieving a letter from infoscore that I have a debt from O2 amounting to more than 500€. I have moved out of Germany 2 years ago and moved back again later last year. When I cancelled my contract, I only cancelled it through phone call. I also cancelled it by the form that they sent to me and I sent back. I remember calling O2 2 years ago asking if I need to do something else and they said 'NO'. Now I moved back to Germany, I am recieving this letter saying I have debt from O2? I cannot reach them by phone call and they don't even have an e-mail contact. I thought there is a Chat system? How can I use this function?