Increase of cellphone contract

I app**** for a contract in April which was on promotion for an iPhone7 and the price was R299 as per the agreement. I only paid R299 in the first month and since then ive been paying R310. I contacted Cell C and spoke to a lady by the name of Hlengiwe who happend to be very rude advising me that prices increased and customers were made aware of this 2 months in advance for July. When i questioned her how is it possible for me to know because i only started paying in June. All in all what i need to find out is how is it that they sold contracts on promotion to people to only pay the agreed price for a month and thereafter increase the instalment. This is actually ***** or was the reason for them having the promotion to trick people into paying more money when they sold the contract at another price. Kindly provide feedback ASAP or i will reverse my payments and have lawyers involved in this matter Email ***