Debiting my account despite the devices being returned

I confirmed a verbal contract with MobilsPlus for an MTN contract for 3 phones. I asked whether the phones were dual sim as this is a must for me. The callcentre agent confirmed that this was the case and we continued with the agreement. Upon receiving my devices on 21-08-2018 I noticed they did not have the dual sim capability. I phoned MobilsPlus and the instructed me to return the devices to the nearest MTN store. I returned the devices that same afternoon and received a reference number for all 3 devices and the 4 sim cards which I returned. It is now 2 months later and after several attempts to contact them and resolve the issue I have been unsuccessful. Today I phoned again and was told the system is offline. I have reference numbers for the courier which returned the devices to the warehouse. This needs to be resolved ASAP as I am not going to allow them to take my money for a service and devices I am not in possession of.