Over billing!

Telkom has to be the worse service provider. If it's not their network then it's their billing! So I have a contract with Telkom for a phone. The instalment thereof is R699. To my surprise, Telkom deducted R1200 for this month. The debit order went through today.Almost double the instalment as per our contract. To say I'm flabbergasted at this would be an understatement. I need an explanation and a refund urgently. I dislike being inconvenienced!