Total lack of service on cellphone repair

The phone in question has been in for repair 3 times already through MTN Presitge and every time comes back with the same error. Every time we call to complain, they refer us to other departments who all say it's not their problem or responsibility. When asked to speak to a manager, we are told every time they are in a meeting or they simply hang up (Tato MNT Repair Centre). We would like a manager from MTN to take responsibility for this query and get it resolved, this has been going on for months now with no success. This reflects very badly on MTN as a company and blaming your service providers who in turn blame you is exceptionally poor. MTN has the SLA with service providers and should stop the buck. We would like to hear from a manager urgently.