contract ended

my contract ended on the 11th of may, and contacted mtn immidiatley to cancel it and they said i will get a confirmation sms, of which i did, now i got a message saying tht i owe R620, what is this money for, what is happening in mtn? how i we supposed to truzt them if they are doing this to their customers? do they expect us to comeback? Mtn is a ****, to think tht i used this network for years, it is very disappointing to experience this