Breach of contract

I have two contracts with the service provider for many years. Every two years I upgraded both my contracts. I received sms's and was also reminded to upgrade. In the beginning of this year, up to May/June I received sms's about my data usage which I immediately checked and purchased more data if needed. During June and July no sms's was received. Suddenly during August I received a sms alerting me that the amount owed on data exceeded R4683.88 which I immediately followed up. I was told that the contract on my cell phone is an \open account."I never changed nor received any sms that my contract was changed. Neither did I get a sms to upgrade my phone. I did not change the contract to an open account"