It all started two years ago when I signed a contract for myself and my partner with two phones. After a year that something in my monthly bill was weird, I decided to check all my expenses and it turned out that I was paying O2 50 euros more than what is written in our contracts. After months of phone calls and accusations from them that we signed for 3 phones. it turned out that they accidentally added another device to our contract that was proven from the store protocol where we signed our contract,  that we never requested, received or appeared in our contract. Following these months of fighting and to many headaches, of course, straight away we sent cancellation letters (per post and online) to both of our numbers and the extra number we received for a discount on the contracts. 
Not surprising at all, seven months after the letters were sent and I was called to be informed that my contract will be cancelled, the end of the contract came and only one number was cancelled.
Guess what? the other two numbers (one of which was never used) started a new one year contract with no possibility of cancellation. I'm already used to calling and accepting the blame from O2 because of course, all that matters is money and not checking where the mistake actually came from. 
So I called and transferred a million times from person to person call after call after call (already three weeks of talking)
The answer from O2 is that the envelope which I sent with three different cancellations papers somehow magically arrived with only one paper inside.
I was not surprised because this is the second time that their “mistake” on me to pay.
But I can’t understand how is it “legal” that details disappear and it is not their responsibility.
Moral of this story, never sign a contract with O2! 
 

Edit: Moved to English Mobile - o2_Kurt