Cancellation is 
 

end of next month, so if you cancel in April, it is end of may.

so, yes you have to pay one more month.

Thank you for the reply.

But my point is I registered “online” last month that I wanted to cancel (through Mein02) and my understanding was this “locked” the date at that point and I had 10 days to confirm it? Oder?

If the cancellation reservation made in “Mein o2” is not confirmed at the telephone number specified there, the reservation expires.
In the case of cancellation in text form, the date on which o2 receives the cancellation is decisive. 

well, i guess the last thing O2 would want to do is make cancellation easy.

thanks.

In fact, the termination in text form is contractually agreed. I think writing a letter is very easy - all you need is a piece of paper and a pen.

The option of preregistering the cancellation online in "Mein o2" and confirming it with a call to the telephone number then stated there is offered by o2 voluntarily and in addition to the contractual provision. The conditions for this - the obligatory telephone call - are described clearly and transparently in "Mein o2". If you do not read or understand it and do not act accordingly, this is not the fault of o2.

The idea behind the phone call is that during the conversation o2 can find out the reasons for the termination and if possible, o2 can sort out these reasons. For example, o2 can offer the customer to move the connection to another address or o2 can offer a faster tariff or a discount and thus convince the customer not to terminate the contract.

Why? It is cleared out, to call within 10 days, not confirming with a letter!

But if you still in the 10 days, call the number.