Happened to me with Celcom 2 years ago. When I was porting out, I settled everything. Several months later they said I have an unsettled amount of around ~rm150. To me it was impossible as I'm postpaid, and that amount is more than 2 months of my bill which I pay every month! Even if I somehow missed the last month before porting out (which I didn't), it's still not that amount. They got a law firm to send letter of demand, then blacklisted me. Incredible.