Over charged account

I have a contract with telkom and end October i didnt pay my account . So on the 25 of November 2015 they debited 794.50 from my bank account. today i got an invoice saying that i owe them 1372.00 and i asked the call centre agent why because i have already paid my account she said she doesn't know yet because the payment i made doesn't reflect on their account and she further said to me that after i have paid that 794.50 i will be owing them 568.50 but my contract with them its for 378.00 and when i asked her why 568.50 instead of 378 i signed for she couldn't give me answers.