Handed over to debt collectors for their own erro!

So back in March 2015 i terminated my contract as the 24 months had ended in February 2015. I struggled to get the termination on time due to technical problems that mtn had at that time. So the new month's billing was done before they terminated and I received airtime , data and sms bundles . I was instructed not to use them ,took 3 days before they effected tge termination and migrated me to prepaid. The data and bundles were taken back and a billing query logged. I was only assisted after writing a report here. End of March i was still debited. I reversed the transaction with my bank and unsuccessfully tried to follow up with mtn on all their channels. I then received smses stating i owe them (2 different amounts ). They never came back to me. Fast forward I've now been handed ovee to debt collectors. There was no communication since April to now when I was threatened by the debt collector who told me my contract ended in October 2015 so MAYBE the fee was for early termination then retreated and said it was an o/s balance. They never respond to email , calls , twitter, nothing. Disgusted at your lack of service and accountability, I am speechless! Not letting this slide!!!