Cell phone contract

I was a client of Nashua Mobile and Altech Autopage took all the contracts over. When I was billed at Nashua Mobile on the 1st of the month I had the opportunity to pay them by the 7th of the next month via EFT. This suited me as may salary date was not constant. Now at Altech Autopage I must my account at the 1st of the month before the 15th of the same month or my services get terminated, and i need to pay an additional R15.00 just to pay by EFT. I am sure this was not part of my contract or terms and conditions with Nashua Mobile, and I cannot understand when they take over a contract that they can change the terms and conditions without my agreeing to such changes. Or am I wrong?