Can anyone please help and assist with this matter as I'm a single parent, student and employee also trying to make ends meet, I've set-up a payment plan wanting to clear off what I owe, but why am I being charged so much extra when this wasn't agreed at

Hereby , I would like to log a complaint regarding the service satisfactory against Vodacom. On the 02-03-2021 they sent me a letter with a balance of r7216.06, which I set-up payment plan of r1000 on a monthly basis. Since March to August I paid r6000 off the balance already and after several attempts if calling these people, getting in touch via email with no response they inform me that I still owe r2098. I called this morning to find out and they stating that my account accumulated interest for telephone and texts messages which I've never received before, I dont want an additional amount added to my account, only to finalize the balance which was agreed a payment plan to be set-up for, how can I go about rectifying this, because it seems like day light ******* for such a huge amount just for text and calls which I never received!