Over Charged

I recently took a contact with Cell C for a new cellphone plus a wifi router. it has been 3 months now, their first billing was within our agreement then out of nowhere today (18 December 2015) I get debited for R2638.20 which is a ridiculous amount that has also left me pennyless, cause I did not sign up for such a billing. Now this has made me lose hope in the way Cell C functions and I'm even considering terminating my contracts with Cell C before I get billed for amounts I don't understand again.