The answer depends on how important it is for you to get out early.

You could simply not pay the monthlies.

They will hunt their money, which will cause additional costs and potentialy a number of entries in your Betreibungsregister, but if you're lucky they'll terminate the contract at some point, or agree to terminate it - tough luck if they don't.

Risky strategy. Again, it all depends on how important it is for you to get out.

If you try the DUTCH way, make sure the contract doesn't start until after your number has been successfully ported.