Payment arrangement not honoured

As of 19 December 2017 I have a payment arrangement of R1,396.10 in place with Dstv u der ref number ********** . On 21 December 2017 I made payment of R1,400. I then received muliple sms messages to say I needed to pay an additional R500. I queried this with the payment plan dept to which I was advised that I had to pay it and they were not interested in my payment plan. I have now been disconnected. Why? I have a payment arrangement and I paid ... disgusting!