Bad service and unfair charge

Yazi with mtn yo can never be happy....... I have a contract with them which was supposed to end in April then I did an upgrade in January 26, yesterday received an email saying that I need to pay a migration fee of R1600? What is it for? I've called their call center 135 no assistance, called an online department but no assistance, called an upgrade line not assisted also I'm so confused and stressed about this.