You can also simply cancel your DSL contract in text form by letter or fax. The cancellation form is available for this purpose (its use is not mandantory). Do not forget your signature. The receipt of the letter or fax at o2 is decisive for compliance with the notice period.

For Flex contracts, the notice period is 4 weeks to the end of the month.

For term contracts, the period of notice is 3 months to the end of the contract term respectively to the end of the extension period. 

o2 does not offer a special English language hotline. Therefore it cannot be guaranteed that the hotline employee speaks English. If you don't speak German, you should get support from a German speaking friend.

If you are intending to terminate a term contract prematurely due to a move abroad, please read the following:

How to do a cancellation of my DSL contract due to moving abroad?

Thanks for your answer ! 

I just did not want to go to the post office in this situation. So I will call first and will send a mail if that does not work. 

Thanks ! 

To send a letter or fax, you don’t have to go to a post office. There are approximately 110,000 Deutsche Post mailboxes in Germany, there is certainly one near you. Postage can be purchased online, from stamp machines and many other outlets. There are also online fax services, so you do not need to own a fax device. 

Wow, thank you so much for your info. Did not know about the online fax thing. It did help a lot ! 

By the way, I just sent it via fax, but will I get a confirmation email or mail as well with fax? 

o2 will never send a fax.

Oh, no I mean, if I fax them, will they send me a confirmation email with cancellation?