Confused and threatened by Mweb

While I was employed by Multichoice we were given Internet devices at staff rates. Just before we got retrenched I started getting billing emails to which I religiously responded and paid the agreed amount. Surprisingly after being retrenched I get sms's saying I owe them, I even responded to one of their emails to ask for clarification but got no response. Now lately I got a very threatening email from their lawyers. Please help.