Telephonic subscription but not cancellation?

I signed up with Telkom Mobile over the phone, no forms were signed. February 2014 an agent contacted me re. renewing the contract, I responded that my connection speed was terrible and will let it run monthly until a better supplier could be found. In December I sent and e-mail to the address used for all previous communication, cancelling the contract. Telkom Mobile debited my account in January and February 2015 which I reversed. Yesterday an sms was received from a collection agency re. outstanding funds. An agent informed after a lengthy telephone conversation that the contract can only be cancelled after completing and e-mailing a form. This in my mind is bullying tactics and unfair. So the contract can be done over the phone but not cancelled??