Only the shop is able to cancel the contract. You should go there even if it is not in the town where you live.

If the shop is not willing to do so, you will have no chance of getting out of it.

As you probably signed the contract there, it is binding - for at least 24 months.

By the way: I don’t understand why people who don’t speak the foreign language well enough don’t ask a friend who speaks the language to accompany and support them in the process of negotiating, reading and signing (!) a long term contract.

@Othon and your really want a new contract with new number or only change tariff with old number?

We have an English section, I moved this thread to it. Your only option is to file a written complaint to Telefónica O2 Germany GmbH & Co. OHG Kundenbetreuung 90345 Nürnberg.

Omikron thanks for the reply. The salesman spoke perfect English with me. Also the paper he handed me did not contain anything regarding "Junge Leute Rabatt" whatsoever. I have a pice of paper with his signature and the 24.99 value. When I went to a shop near where I live I was told that many shops are tricking customers like this on purpose. 

The question is: he has my Ausweis. The system has my date of birth since two years. How come he can even create such contract?

It does not make sense to make another trip just to cancel this. It's inadmissible that a big company like O2 cannot solve this situation. As much as it is a shame that it has such people representing it towards their customers. 

Right now I just want to undo the damage. Won't mind returning to my old plan, and, depending on how much effort I have to put into solving this case, I will get out of O2.