Terrible service from MTN

I have been an MTN customer since 1998 with the same cell number. Since January I am UNABLE to receive calls on my MTN number. I can make calls but when someone tries to phone me they get "Number not available". I have contracted MTN numerous times, been into there stjores a few times and nobody can sort this problem out. I was about to get a new contract with MTN but now am changing service providers.