Unauthorised debits, account handed over

I had a Data contract with Cell C for many years where I had very little problems with them, untill I requested to cancel my contract. For the last five months of my contract they started to debit my bank account with much higher amounts then agreed on and much higher than my monthly premium. I tried phoning them, but they failed to provide an sesible explanation as the why. I loged a complaint via email and NEVER received a response from Cell C. I proceeded to reverse the debit orders in the hope that they will cantact me so we can resopve the issue, but that never happend. Cell C handed my account over to debt collectors who phone me, I explained my problem to this person and asked for Cell C to contact me to explain why the higher charges and to have this resolved, however I have now received a sms informing me that a tracing agent had neen assigned to my case. This is really really bad. All the years I paid my account, now I seek an explanation for additional cost not agreed on and this is how they treat me.