12 months plan changed to 6 months

I have change my payment plan from 12 months to 6 months payment with a concept of i will never pay interest but they decided to make it double 12 months and 6 months plan,I realized this year that I've been paying more money than the one i should pay. so i tried to call them to tell them to cancel all the plans that are not suppose to be on my statement. i even asked to speak to the seniors but they put me on hold for 30 minutes . So I'm so angry with they're services. And I'm not going to pay my account until they fix it.