Terrible customer service and Harassment

So in the beginning of March 2017 I visited a Mr Price store to change my account from a 6 month to a 12 month as i was going on maternity leave from the 1st of April ,which means I'd only be getting a percentage of my salary. I explained this to the lady and she made me fill out a form and faxed it and said it will take a month for the change so it should reflect for my next payment. To cut the long story short it never ever changed which left me in a financial mess as i couldn't afford the high installment. I ended up making an agreement to pay R500 per month but as soon as i pay I've been getting calls non stop about why i'm paying short and when will my next payment be! Common sense I've just paid how on earth can you ask when am i going to be making another payment obviously it will be the following month!! secondly the account went into arrears because of your staffs negligence the forms were all filled in and signed but never got captured clearly and I've been pleading my case to every damn person that calls me to harass me about payment. I can understand if you call for non payment but not when payment has been made its ridiculous!!!!! absolute terrible service!!!!