Double billing

I received my Altech statement and it was at least double what my average bill was for the previous two years, on the same contract. And I know for a fact that I didn't do anything out of the ordinary with regards to phone calls or data usage. This month they are charging me R653, where my average before that was never over R280. On my invoice it looks like they are charging me for two devices. Altech renewed my contract without my permission during April or May, I never received a new device, I just started receiving a larger than normal bill. My average bill before May was R270. May they charged me R372, June R454, and now R653?!?! I've called them numerous times, I've filled in complaint forms on their website, I've been into the store, and nobody is sorting this problem out for me! Extremely bad service!