Customer worthlessness

I received feedback from Vodacom. The amount they were billing me of R1655.00 was for 4 months handset instalments that they forgot to charge me in 2013. Now they found it out 2 years later and there is nothing they can do. I have had my contract with them for 16 years and in the last 2 years have paid them more than R15,000. But still there is nothing they can do. Oh I lie - I can pay it back over 4 instalments. I am going to take this further as I don't believe that I have to be punished for someone who did not do their job. My current contract expires on the 2/9/2015 and I will not be renewing it. I am going to Telkom direct at least with them I won't be a cell number. I do not want anyone to call me and I do not want anyone to ever send me any promotional emails or sms's unless it is to tell me I don't have to pay this amount.