Dear Sir or Madam,
as far as I understand your question, you want to know if the 135 Euro are a justified claim and if it´s better to pay the money to avoid being sued.
Please note: Adding or leaving out information can completely change the given answer and legal advice.
To the problem: If you can´t prove that you properly closed the contracts through documents (or witnesses), which I guess from the circumstances of the case you cannnot, you should pay the claim.
By this you avoid to have to pay the court fees.
Because the burden of proof for the cancellation of the contracts is on you, and I guess that the subject terms of contract of O2 state that you have to sign or send a certain document you called
there is little chance for you to win this law suit.
But I would strongly recommend to see a lawyer and show him all the documents you have in this case. For a first legal advice you can negotiate an adequate fee with most of the lawyers.
I hope to have given you a first general view and please refer to the possibility of specifying your request on "frag-einen-Anwalt.de" or contact me directly by the "Online-Anfrage" or in my office.