Incorrect deduction of December instalment

DirectAxis decided to deduct my instalment earlier in December on the 15th, instead of on the 20th. Because I use Debicheck, the instalment was deducted as soon as my salary was paid on Friday, the 19th. I have since before Christmas been harassed on a daily basis with phone calls threatening that I will be handed over if I don't pay. I have even submitted bank statements to show that the money went out of my account. The call centre agents are useless and cannot help with anything except saying my account is in arrears. It's not. I don't get any reply from the Collections department via email and trying to phone them is a useless exercise. The fact is - the money was deducted from my bank account so as far as I am concerned, I paid my instalment. Please stop harassing me and fix your system. In future, also please don't assume that I get paid earlier in December. My salary date stays the same.