Karolina, this is another world-wide problem - the contracts are long-term (to the advantage of the network) and automatically renewable. I have had a similar problem when moving from Bulgaria to Germany and (ridiculously, but not for me) when moving back from DE to BG. For M-tel, the lowest cost/harm solution is to skip paying 3 monthly taxes, which invalidates your contract and then pay them right after your contract is cut, but before they report you to the credit register or sue you. Usually, you would have more than enough time between these two events, but it would be best if you consult (or have a friend do it) with an M-tel employee - that's where I learnt that 'secret'. Unfortunately getting out of O2-DE is not that easy at all...