Charge more than the agreed amount.

I've signed a three year contract with Altech Netstar to install tracker in my car. The agreed amount was R101.00 per month ,late 2014 Netstar personnel called me to inform of extra benefit they wanted to give me. I asked them over the phone if the premium will be increased or not and if that was case I was not interested as long as there were increment on premium. They told me the premium will stay the same no increment will be added. On my surprise end of November 2014 the premium increased to R 346.00 and I decided to reverse that amount from my bank account, because it was a breach of contract we never agreed to R346.00 or any increment.They keep on phoning every now and then. Please help on this regard.<br> My contact numbers are ***. I do not owe them any money since they breach a contract.