Sharing the same cell number with another

I have signed up for a contract and up to now I have not used it since August 2015 due to an error that has been made by MTN data captures or who ever capturing the customers contract information on the system. My SIM card is failing to register on the Network due to that I and another person we are assigned to one cell number which now makes me to be offline the network. This is the totally unacceptable, I called call center and I went to MTN shops they don't help up to my satisfaction because the problems is still existing. I'm frustrated and I need any advise on what to do. They also keep on debiting my account for what I'm not using so what am I really paying for.