MTN increases monthly payment

I took out a One-to One contract via Foshini Stores from MTN, a few months ago. Then I received an sms to say the prices are going up on the contracts. I visited a Foshini store on 1 May 2015 and they phoned the One-to-One Department on my behalf to find out about the increase. I spoke to them, they of course stated that the phone call is recorded! During our conversation, I inquired about the price increase, as I have entered into the contract exactly because of the amount I could afford to pay. They assured me not once, but repeatedly that the One-to-One contracts are not being increased and the amount I am contracted to will remain the same for the duration of the contract. Then on 1 June I received a letter from MTN One-to-One that the amount I will have to pay is increasing. Although they also stated that the amount of airtime will increase as well, that IS NOT THE POINT! I chose the original amount because I have to budget and that is what I can afford. How can I get that recorded telephone conversation to prove that they assured me the contract amount will stay the same? At the least, it is dishonest?