Shocking *****

I had a contract with MTN that i converted to pre-paid when the contracts term expired. I received an sms from a debt collecting agency after more than a year. When i enquired they told me MTN handed me over as the account was at Legal. My question to the consultant was how could the account be changed from contract to pre-paid if the account was not settle. She told me it is not possible as this service is only available for the last year. That is strange as i am using the same MTN number as a pre-paid number for the last year and a half. They are now telling me that the account was still billed while it was on pre-paid. Now how can you bill a pre-paid number. This is shocking