Staff at branch level gives misleading information

I bought a cellphone in beginning of march this year and was advice when buying the cellphone that when buying a cellphone it can only be put on a 6 month plan, so looking at my statement this month I see on the cellphone plan I still Owen R1100 but August is suppose to be my last installment on it as in March I paid my 1st installment on the cellphone. when calling the customercare service and explaining my concern she consultant micaela put me on hold of almost 10 mins to have a look into my query she comes back and tell me that the cellphone is a 6 months plan with no interest each month and the 7th month you pay the interest which struct me at surprise cause when you buy a item on credit and it's on 6 Months you pay 6 months only with interest, I feel like I'm being ****** in some way and I'm not happy with this information