Living in Europe, I couldn't believe that if I wanted to unsubscribe to New York Times, I would need to call one of their hotlines which operated in US time-zones. IIRC the open hours were after midnight in my timezone, and their local hotline was out of order. I seriously thought that I had signed up for a phishing site ...
It's just as bad in Europe! Signed up to O2 Deutschland - had to send a fax or send a physical letter to cancel.
Your comment below says that there is no receipt for confirmation. O2 provides a default PDF form on their website which to fill for termination. The letter explicitly states that "o2 should send you a written confirmation of cancellation". It is illegal for O2 to be in receipt of a letter and not send a confirmation. I am sorry if that happened to you!
Don't get me wrong - the auto-renewal of contract practices in Germany are predatory for the consumers. Recently, there has been a change in law that forces providers to extend contracts by 1 month instead of 1 or 2 years.
[0] https://www.o2online.de/service/kuendigung/
[1] https://en.avm.de/service/knowledge-base/dok/FRITZ-Box-7490/...
[2] https://www.deutschepost.de/de/e/epost.html
[3] https://static2.o9.de/resource/blob/498264/12cd6ca6ee17a02b9...