Mtn ***** from their customers

I trusted Mtn with my personal details,after the end of my cellphone contract l gave them a notice of cancellation and they accepted it,Boom!! after a year they send their lawyers MBD after me saying l owe them,they have been adding money somewhere every month as my debt Is this how Mtn is makjng money? ls this how they really treat their customers? or l am the unlucky victim