3rd month with no device and MTN still has no resolution!

On 26 June we returned a Huawei P8 lite which had become faulty after we took out a contract in May. We were told the MTN policy is that the phone would be repaired or replaced within 21 working days. Here I sit on working day 57 without a device. This is COMPLETELY unacceptable. For the first month, the device did not even leave the store, and then, the device was collected by the courier but never received at the warehouse. Our query has been escalated to the Regional Manager, however we're not having much luck with him either. He is not giving us the service we require, yet he cannot provide us with any details of anyone who could better assist us. Continually telling us it has been escalated and is waiting for authorisation. I've really had enough. It's nearly 3 calendar months without the device. Anyone with any advice on how to resolve queries with MTN please point me in the right direction. Feeling that changing service providers was the worst move I could've made.