Poor service at telkom

Have a contract with Telkom, and only paying 299 per month, however for some reason not known to me, was debited 749 in december month, and in january got 1850 that i had to pay, from a monthly debit order of 299 i am now supposed to pay over 1800 for what i don't know, tried calling the call center however my calls were all dropped without any assistance, poor service