Bad services

I'm so disappointed with this company. My sim which I've been using since 2013 was re-allocated to someone else without my knowledge. Paid up data and airtime contract I had with MTN last month and was told I don't owe anything. Now my sim is not working, after emails and calls and not getting any assistance, was told today my number was re-allocated in October. How do you re-allocate someone's number in less than 3 months??!!!!